1) The initial creation of flash cards is good for cementing a little extra information into your memory as it’ll be a form of repetition. In the long term you can re-read your flashcards, or test yourself e.g. ask yourself a question relevant to the flash card you’re holding and then see if you can remember the information you’ve written down. You can do this with friends too and make it into a small quiz.
2) No, extra reading is not necessary. It’s helpful if you need further clarification on a topic, but for the sake of passing your exams, it’s not vital. Everything you need should be in your specification and official exam board textbook.