There is a puzzle in the original Portal that you can solve by stacking up a bunch of cameras. For the longest time, I had always done this and never attempted to properly solve the puzzle.
Somewhat surprisingly, I’m not able to quickly find someone doing it on YouTube, but it’s the puzzle where there’s a cube propping open a wall panel where Ratman was. There’s several cameras in there. If you take all of the cameras and your cube, you can basically make some janky stairs and climb your way up to the next section.
(TL;DR):Start by playing/replaying casually, eventually you'll feel like you can beat it in 1 sitting. Time yourself, and then try to beat your time. As you get more and more comfortable, search for tutorials and incorporate more and more tricks/glitches in your runs. Eventually you'll have a trick for each chamber, and your runs will look like mine.
It started with me just replaying the game occasionally, and after a year or two I thought hey, it's pretty short, let's just see if I can beat the whole thing in one sitting.
My first run took several hours because I didn't have the solutions memorized lol. The next day I decided to time it, and since a lot of the solutions were fresh in my memory it only took ~2hrs. So I just kept telling myself "I can do it faster than that" and I kept doing it again and again until it only took me about 40mins.
At some point I looked up speedruns and found Noircat's run at GDQ, and it blew me away. I wanted to be able to do that too, so I watched a bunch of tutorials and started learning some tricks (this was one of them, can't find the others... these days we have a Google doc). I started out with just a couple, but they all took a massive chunk out of my final time. After a lot of learning and practice I had a trick for nearly every chamber, which got my time down to ~20 minutes. After that, it was a process of watching other people's runs and saying "I could probably pull off that trick / learn that glitch" and just replacing old strategies with new ones.