How do I make my own internet?

That's called an intranet (which is like the internet but for a building or small area).
The internet isn't really anything more than computers connected to other computers. If you want to make your own internet you can connect two computers whether wired or wireless, and voila. You can host a website locally that only those two computers can access.
See also: !selfhosting@slrpnk.net
- You can even set up your own DNS and create your own domain names for said websites.
- Don't you need at least AS numbers and BGP (or equivalent multiple independent networks) for it to be an "inter"net?
  
  If you run BGP, yes. Instead, you can always just build up huge-ass fixed routing tables that are impossible to maintain once you get more than a handful of networks....
- I've read a creepy pasta somewhere from this guy's pov, that somehow discovered a computer at one of the businesses they took over as a contracted IT, think car wash or something, and they've ended up discovering a computer in a closet that was networked with other computers in the area to share CSAM and other smut. I forgot how the story ended as it was fairly long, but they were describing the technical non-sophistication of the whole network in great detail. The PCs were basically just networked windows xp workstations or something, without any sort of password protection on them or their shared folders.

Does your computer have a floppy drive? You might be able to find a copy of this at a secondhand store:
- OK. Now how to make a floppy drive?
  
  You can use this design, but if you want it to accommodate 3½-inch disks you'll need to scale it down to two thirds of its specified size before printing it.

Take two wireless access points, connect them together, tada, you have an internet....
The thing is "the internet" is really only the sum of its parts, it's interconnectivity is what gives its intrinsic value.
- Or a LAN. Could do a WAN which itself can be interconnected over a wide area. Usually by routing over the internet but you could use something like satellite uplink or miles of dedicated cables.
  But the interconnection of multiple LANS and WANS is what would make an “internet”.
  So maybe 2 universities joining their own networks would be moving towards a private “internet” but I think we’d still call that a private network or a WAN.
  It’s interesting to consider where the definitions change.

Here's how to do it in 6 easy steps.
And the end result:
- Do the Elders know you have this?!

With hookers and blackjack?
- In fact, forget the internet!

Could you give more details about what you mean by "homemade internet"? The first thing I thought of was just a LAN or Intranet which I see many people have already suggested but I feel like that may not be what you're actually talking about
- This could have been a really interesting question if OP hadn't been so vague. As is, there's too many interpretations to answer. Do they mean the physical connections? The protocols and services like IP, DNS and BGP? The world wide web, with its sites, links and search engines?
  Does OP consider the Dark Web its own internet? Or a large corporate network its own internet? What about self-hosting a huge number of services in your own home?

Take two computers, connect then via router, finito.
- You need two entire networks.
  So yeah, take two computers, set them on different networks, bridge them via router.
- Connect them directly via wifi.

I think what you're talking about is called a "LAN"/"Local Area Network".

Well, the Internet is connected together using routers/switches. Your own home network is a "private internet" until you pay to connect it to the big one. So if you want, nothing is stopping you from running cables to your neighbors and hooking together. But then you won't have access to anything useful except whatever servers you guys run
Was that your question?
- I was going to make a joke that you didn't even mention the tubes, but TIL that's anti-net-neutrality idiot propaganda :( Idk why I thought it was, like, Tim Burners Lee or someone trying to explain things to non-nerds.
- Nitpicking here, but unless there are multiple sites, that's just a LAN (local area network), and even across multiple sites without certain other bits and pieces, it's still "only" a WAN (wide area network).
  Admittedly, the latter can be called an internet (lowercase i important). The LAN would be an intranet.
  To replace the Internet (capital-i and all) you'd need, at the very least, a DNS server of some sort, if not reimplementation of any tools that assume IANA and ICANN are at the top of the hierarchy (or successfully lying to them about where those roots are on the network).
  To head off the question of where VPNs fit into all of this: Any or all of this could be implemented over a VPN that uses the existing capital-I Internet to transfer data, at the expense of some bandwidth.
  Some care would be needed to not let data leak onto the real Internet at any of the end points, but it could be done. This is basically what TOR does for the Dark Web.

North Korea did that

This could mean many things.
One interpretation is to re-invent the world wide web. One recent effort to do so is Gemini, which is pretty charming with it's completely text-based tiny internet that feels like the intimate web WWW must have been like in the early 90s or something.
If I had too much time on my hands I would love to tinker with it.

https://blog.thelifeofkenneth.com/2017/11/creating-autonomous-system-for-fun-and.html?m=1
It's doable, just needs a fair bit of hardware, time and money

Can you give an example of a possibility you think of? In its simplest form it's exactly your house wifi if you disconnect your internet uplink. Anything bigger is also exactly a subset of the current internet disconnected from the rest, plus you having to maintain infrastructure.

Don't need to image. I have run an ISP for near a decade and know how.
If you genuinely want to I would say have a look into Cisco or Juniper service provider certifications. Then look into how to run your own authoritative DNS server.

Here ya go.

@See VPC, VPN, LAN, etc...
Corporate intranets (intranet essentially being "internal network" while internet is essentially "global network") have existed for a long time.
But there are intranets all around us - everything on your side of your router can be treated as a local intranet and you can even set up a local DNS server (this is commonly used for ad blocking and I believe pi-holes use this technique though they might just use address blacklisting).
So, TL;DR you already do! A hacker is you!

I know this isn't what is being asked, but has any of us considered a crowd network where independent users mesh? I would say that I would not want the mesh to meet the www due to endpoint abuse, but to have an independent can/man could be pretty rad.
- Check Lora Radio mesh. It’s kinda what you describe.
  
  I've been interested in this for a while.
  I'm in a regional area of Australia though, so can't get a connection out here.
- I mean you could have an open wifi mesh and/or a network of either cheap fibre/ethernet with open switches. Then using OSPF or a similar routing protocol that supports routing over LAN networks you could handle the routing between all the remote networks.
  I think you'd need to break the network up at some points to break down the broadcast domains. You could do a similar thing to defederating, by not accepting certain routes, or routes from certain OSPF nodes.
  Issues with LANs that get too big without splitting into a new LAN (limiting broadcast domains) and definitely even the most modern wifi becomes problematic with a number of active stations online (wifi is half duplex in operation). So multiple channels and some backbone either over point to point radio links, or cable to connect wifi zones and alternate channels would improve things somewhat.
  Not sure why you'd want to do something like this. But the tech to do it is fairly inexpensive.
  
  Not sure exactly what I would do with it either. It's not meant for illegal activity, but I can see it being used they way. Properly secured it could be another dark web.
  I thought about it needing some kind of border protocol to manage locations and routes. One would need to come up with an address and probably a certificate.
  Regarding physical connections, I would say it's easiest to use wireless. I personally wouldn't want to advertise that my location is my network, so like the onion network I would like it to route away and double back.

Use an overlay network and run dns on it.

You mean like an independent mesh?
I thought about that as well, some sort of "pirate radio"/mesh/distributed wireless where one could host a website kinda like the Internet of old and did some light research, but all I could find are Meshtastic (Messages only, no internet) and LoraWAN (seems more focused on IoT)