I applied to coach at my gym last March, started training in June, and became an official coach in September.
Due to my age when I started training, I was not allowed to spot, so I learned most of my spotting from either asking some of the other coaches or looking up tutorials on YouTube.
For spotting bigger athletes (as someone who is not super strong), It was just a lot of trial and error for me. A lot of the things that made it harder for some of these athletes (or any athlete in general) I have worked with was strength and the mental aspect. Once these things were worked on, It made it 10x easier to spot them.
For knowing the corrections, it wasn't super hard for me since I have been a gymnast for 10+ years, so most of the stuff was things I was told when learning the same skills. For skills you don't do, I would suggest looking up youtube videos (or tutorials with skill breakdowns) to know what the skill is supposed to look like.