What advice would you give to someone starting a business?

Get an accountant.
Keep your business bank account separate from your personal bank account.
80% of small businesses close within a couple of years. If that happens, try not to take it personally, it happens to most.
Sometimes, saying "no" to an opportunity is necessary to give you the chance to say "yes" to a better opportunity that comes later.
- That last part is tricky, after all there's also a saying "a bird in the hand is worth two in the bush", it's a balancing act.
  For example most of my business comes from repeat business customers and it's definitely underpaid compared to what I can get in different niches, but on the other hand I get regular jobs from it. My colleagues focusing on the better paying niches are always hunting for the next gig and don't know where the money will come from next month.

Business owner or not, always salt your pasta water.
- But which salt? I use a mixture of Arabian Sea salt and Himalayan rock salt. Some people like using salt sourced from Atlantic Ocean, as it is considered to be saltier than salts of other oceans.
  
  Potentially unpopular but I don’t think it matters for pasta water.
  I’m not that deep into salts but I keep a few on hand. The standard diamond crystal/maldon for cooking, as well as an unrefined sea salt for the same purpose. The standards are standard for a reason and they’re more than enough for my non-chef preferences, these and a random sea salt for the grinder are what I use 90% of the time.
  I like fleur de sel or flor de sal for finishing, though I can’t tell the difference between the two (I believe it is region, but my palate is far from capable of differentiating much). I have a sel gris that came with a salt set that is meant to be used as a finisher, but fleur de sel is more popular and thus easier to restock. I use black Hawaiian salt as a finisher for Hawaiian dishes. The black salt, unrefined, and fleur de sel are good for eating the salt alone which is a guilty pleasure.
  I’m actually pretty surprised to hear that some prefer saltier salt. The chefs I have asked like lower sodium and higher mineral contents because they have more flavor. That said both tabelog gold/*** sushi chefs I’ve asked heat the salt to remove moisture, which then increases saltiness by volume, so I guess I’m not that surprised. I do this for sushi rice for authenticity but like I said, my palate is solidly mid and I can’t tell.
  What do you like those salts for? I’m not really a salt enthusiast and just use what has been suggested by the chefs I like. Don’t think I’ve heard of Arabian Sea salt and I’ve never used Himalayan myself, but I’m very much interested!
  
  Mono sodium glutamate

Be fair to your customers AND yourself.
Don’t bend over backwards to satisfy customers who cannot be satisfied. Some people are just miserable human beings, and some will gladly bankrupt you if it saves them a nickel. Is their repeat business the kind of business you want?
Don’t be afraid to say NO or to set firm boundaries. Nobody respects a spineless pushover.
- I fired 2 customers last week and my crew would follow me into hell because of it.

Register your company and brand in some tax haven. Have a subsidiary “rent” the brand for the amount of income you make, so you have no tax to pay.
Do not do this. It is illegal. But starbucks and other companies do
- Donald?

Don't start a business without a plan to handle all the stuff that is not your product.
If you are good at whatever your product is you are not automatically good at building and leading an enterprise (a company), and that may destroy your ambitions... In other words: Even if you have a very promising product you may fail due to completely unrelated organisational hassles because starting a business will drag you into processes that will drain ressources and your brain for completely different stuff, be it financial, legal, hiring and firing staff, customers (customers...), renting, ordering, offering, paper works, ecology and what not.
This shit can and will hurt, in the core meaning of the word, if you are not prepared.
- Yep this is the reality. I was a small business owner before when I was young and I was decent at what I did but I had no idea of the realities of running a business. I did 10 years in corporate before starting another one and it's night and day because I now have way more business context and can breeze through all the side tasks with an understanding of how successful businesses are actually run.

Never run a business as a worker making ends meet. You're one recession away from failure. If you are not putting back a healthy margin to build a business, save for the unknown and opportunities, you're in a bad place that will likely bite you in the long run.
Do whatever it takes to put off hiring people as long as possible, and then push way way past that. Never hire people unless forced to take loads of money in exchange. Your efficiency will drop drastically and they'll be worth a tenth of yourself for half of your money.

I have a few things.
Know when to fire customers
Know where the money is coming from. Hope can’t pay bills
Hire only good people for your core staff

Not me, but I heard a good line from a sign maker.
They said consult the local sign maker. That person knows a ton about businesses opening and closing, and which locations are cursed.

Cover the product in camo, retweet some racists from your official account then just grift your way to retirement.

Don't get into business with a narcissist. If you don't figure out they're a narcissist until after the business has started, bail or kick em out.

Learn the basics of financial accounting, even if you employ an accountant.
- the local Community College has Accounting 101 available for cheap, and it's all mission critical information if you want to run a business..

Pay your damn employees

Just keep at it. It doesn't have to be profitable in the beginning but if you enjoy it, you will excel in it.

Deleted
- An exit strategy can be who carries on the business after you retire - if you find a niche where people need you, a business can last and grow.

I don't run a business but worked at a small company where the owner was the founder. He told me running a business is doing what you're paid to do about 30% of the time and boring administrative stuff 70% of the time.

Setting up an LLC or S Corp or whatever kind of entity makes sense for your business is easy to do. Don't use legal zoom. Be your own registered agent. Just be sure to do your quarterly or annual business filings (depending on industry).
Get a CPA. talk to them about what you spend money on.

Read Kahneman and seriously consider not doing it unless you are a flaming optimist that just can't be held back.
- This. The best business advice I can give as someone who started a business is ... don't.
  The odds are terrible, you lose your soul to compromise, and ultimately end up selling out or going poor; either way the project is dead or milked by the buyer and you either have money from the sale or no money at all. 99% chance of the latter.
  
  There's a very good reason your business failed tho 😂

Business is easy to run and even easier to run into the ground. Focusing on not letting bad decisions fuck up everything is your #1 priority.
Most businesses shut down for the stupidest reasons, and it always stems from one small bad move that wasn't/couldn't be corrected in time.

Make sure you are registered with the correct tax offices for your location. Where I am, you have to register at the federal and county level (the county communicates with the state). If a brick and mortar business, then I would assume you also have to register with the city, but I don't know.
Also, make sure your policies are explicitly written. Preferably multiple times. And maybe even verbally going over them....some clients need more "direction" than others.
If your business is multistate or multinational, make sure you know the laws there! Most US states have minimum limits before you have to pay taxes. Every country has their own policies on these things and paperwork to fill out (some easier than others). There are companies that will collect and remit taxes for you, which can be extremely helpful especially as a small business in multiple jurisdictions.
Also, different jurisdictions have rules about what can and can't be sold. Sanctioned countries, rules about children's toys, varying levels of legality for different substances and materials...just things I can think of off the top of my head.
It can get overwhelming and business/contract lawyers are underrated and not as expensive as someone might think (depending on what you need).

You have no money
- Take out a loan to start the business. It's what my brother did. He was months away from declaring bankruptcy. Now he owns a very nice house, and had a successful business.

Establish your own discipline with top priority, even if it means doing things in an inefficient manner. The most important thing is to avoid the lapses in functioning that come from things breaking down or from your own discipline flagging.
As they say “a bird in the hand is worth two in the bush”. To expand on that a little, to keep the bird in your hand you need discipline, and to get the bird in the bush you need daring.
When starting a new business, daring is in abundant supply, and inspiration performs the role of discipline. But in order to succeed, you need to develop discipline before the inspiration wears off.
A machine at 10% efficiency, that’s running, is worth more than a machine at 90% efficiency that isn’t running.

The less shits you give trying to "be a real business" the better.
For example a "real" restaurant would have a hostess, perfect china, perfect decor, etc. We joke that we don't have a "real" restaurant (our friends and neighbors say the same) and it took me a hot minute to realize it's all about the food & people, if the glasses have water stains, if there is a plate with a small chip it isn't the end of the world. My background in fine dining still makes me twitch every now and again but I get over it quickly.
Personal anecdote: we opened our restaurant 4 months before covid shut everything down and we found out real quick what was important: keep as consistent as possible, keep yourself & partner happy, know when to shut it down & take a break.
This is our 5th year, it's just the two of us with one PT server in the summer, we know we aren't going to be independently wealthy but we make what we need, don't carry debt, go to the beach every day in the summer (high season), and enjoy our neighbors & friends when they come over to eat (especially during the low season).

(experiences from small scale agriculture)
Things will always cost more than you think.
Learn about book keeping, even if you don't start out doing your own book keeping, and do your best to maintain good book keeping practices... the easier you make it for an accountant to look for tax credits, profits, losses, depreciation, and potential write-offs the easier it will be to spend money paying an accounted to make sure the stuff is done correctly. And if/when you start doing your own book keeping and working on summaries if definitely helps to mentally make sense of what's going on in your business.
If you only have one source for some vital supplies or service, its a very good idea to be on the look out for alternative sources and be ready to find substitutes for those supplies/services.
You get to say "no". If you only want a small shop that does business with a "X" amount of clients/customer, you are under no obligation to try to provide goods/services to "X + N" amount of clients/customers. So long as you can work sustainably, its not your job or purpose to supply the entire "market". If there's more demand than supply, its okay for some other person to start their own thing and serve some of that ample demand. So when people start to complain that you aren't "big enough" or that "you could have a bigger business" when you aren't interested, tell them "no."

Failure is part of the growing process. The probabilities of your first company being sucessfull is very slim. But a failure helps you with the next company you create.
Source: started 15 different companies and 2 worked out. But they really worked out.

Once you’ve hired people you need to guard the coffers. Always make sure you know how much money goes in and out of the business. You can’t trust anyone for 100% when it comes to money. You can trust some people for 90% and a select few for 99%, but never 100%. Since you can’t read peoples’ minds. You don’t know what’s going on in their personal life. Like they could be gambling addicts and are in debt to a loan shark and you wouldn’t know until it’s too late and they’ve stolen a shit ton of money from the business. Lots of businesses have gone down because of a rogue employee.
Also have a policy that you never ask to transfer for a large sum of money trough email or the phone. Even big companies with a financial department full of registered accountants have been duped into transferring millions to a scam artist. So it’s probably better that you, the business owner, are the only one who can transfer or pay for bills that are above a certain threshold. Even better to have multiple business accounts, one is to pay the regular bills another to pay for the salaries and a savings account where you transfer the profits to. The last one only you have access to and each quarter or half year you transfer just enough money from the savings account to the other accounts to pay for all the expenses.
And be wary about emails and letters of your suppliers saying that they’ve changed their bank account number always double check with another source.

Spend a few hundred dollars on a lawyer / accountant to make sure it's set up well in the beginning legalities wise. The money will come back to you almost immediately and it can potentially save you absolutely massive difficulties.
Track metrics, have a list of the main stuff you're keeping an eye on and what your goals are for it. It's surprisingly easy to get distracted and work on the fun tasks instead of the tasks that will grow the business (I mean, you need a mix of both, but track your metrics and be honest about what areas are working and not.)
Take care of yourself. Sleep, exercise, don't get stressed, take breaks. You being in good shape will grow the business; you falling apart will tear it down even if you hit your goal for the month in your work output.
Bring the love. It'll make what you're doing more enjoyable. Do a fucking fantastic job on stuff because it's enjoyable and pride-inducing to do that; that's actually easier to do that way than forcing yourself through a crappy job because you have to do this thing for this task.