Short answer: the internet
Long answer: I did a lot of googling, and watched YouTube videos. This is like the only tutorial I watched to learn KiCAD. Jeremy Blum also has a good set of videos for learning Eagle. I personally learned that I prefer KiCAD over Eagle, since it's all free, easier to use, and doesn't have any limits on board size unlike Eagle (which has a free version but it's limited.) Overall it's just trial and error, googling, watching videos, and asking questions online whenever I get stuck on a specific thing. You can also order books from amazon but I don't think it adds much more value than the internet.