Read the rules. Write it down. Know how your character works. If you can't play your character, a new, simpler character will be provided for you so you can stop wasting my time.
I've been playing these games for 30+ years now, and I have never met anyone who could learn all the rules that apply to any character more complicated than, maybe, a 1E fighter. Heck, I don't think I've ever actually seen anyone learn all of those, if you include speed factors and the special cases in the number of attacks rules. I mean, sure, you can be basically familiar with them. But you're not going to be able to remember, with perfect reliability, all the edge cases and questions about phrasing that might apply. Especially not for spellcasters.
So, okay, "write it down", but if we're writing it down, why not just use the one that was
already written down, which is to say, the book?
I mean, yeah, I can have a pretty good idea of how most of my stuff works, but if I'm playing a caster, and someone asks a question about the boundary cases of one of my spells, I'm gonna look it up. I am not going to have 100% confidence that I remember the exact wording of how teleport familiarity is defined, or the exact restrictions imposed by a grapple, or whatever else.