Good question!
A mixture of watching videos, reading blogs and consulting fitting books. And taking part in Facebook groups.
And doing a couple of fitting courses.
So, I'd really recommend looking at In-House Patterns. Alexandra is really good at explaining how and why you might an alteration. She has many videos to explain certain techniques. She also has a long course and if you can afford it it's very good. I learnt absolutely loads from doing it.
Fitting books such as Real Fit for Real People is good. It might look old-fashioned but the skills are still completely relevant.
Threads Magazine has an archive of articles which are very informative.
A lot of it is understanding your own shape and what fit means for you. So for example, for me, I can see that the darts are just too high. That's a measuring error by me, and I know I need to lower it. And then I also know that darts should end an inch or so from the apex/bust point and this one is a bit too long so I'll relocate the bust point, pull the dart point back and then redraw the dart legs.
Learning about darts is a valuable skill: you can rotate them to almost anywhere from the bust point, you can split them and have more than one, you can turn them into a Princess seam. But unless anyone tells you this and shows you how you're always stuck with something that might not work best for you.
Mostly though, I just read! And pick things up.