I think making a few flashcards wont hurt, I too am a year 12 student and have been making them consistently since the start of the year! It has already saved me, I've had many exams already, each of which I did quite well due to my flashcards! But when making flashcards, yes use the textbook, but make sure to include as many exam styled questions as you can from pmt or something, and if there are some exam styled questions which you don't understand make sure to ask your teacher about them. Obviously flashcards aren't everyone's cup of tea, but I'm simply carrying on what I did for gcses which I found worked for me the best. You may find a different revision method works better for you!