Where shall I get the information to make flashcards for GCSEs?Watch this thread
The specification so that you know you’re testing yourself on what’s necessary in the topic. The text book so that you can find the required information, and learn it. You could also make flashcards from past papers on questions you get wrong.
How are you going to achieve that in a way which works for you? If you do an exam right now, why aren't you getting good marks? What is it that you haven't learned and need to? Those are the things you put on your flashcards, assuming they're facts (if they're e.g. maths techniques then flashcards will be about as much use as a chocolate teapot - you need understanding then, not rote learning). It doesn't matter whether you get them from the spec, your textbook, your class notes, exam paper markschemes...