The problem with flash cards (as I see it) is that I don't see how they can do anything but help you memorize stuff. I don't see how they can help you understand stuff. There will always be a higher information compression ratio with understanding than with memorization. If I understand a principle, I can "decompress" it into an infinite amount of facts. If I remember a fact, then I merely remember a fact. That's it. So I'd need infinite memory to memorize enough facts to match understanding one concept. What a waste of time.

Here's how you study math.

Do problems.

When, in the course of doing a problem, you run across the need for a formula or theorem that you don't remember, derive or prove it. If you can't derive it, go to where the book derived it (if it didn't, get a better book) and follow along. I mean act like you're reading it for the first time and fill in any missing details and write the derivation out in full. Do this every single time you don't know how to derive something.

Soon you'll be complaining about how you're slow and stupid just like me.