(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.