Read from class notes and textbooks to acquire a basic knowledge of the subject. It is tempting to read while listening to music or with the TV on, but these distractions are not conducive to effective revisions. Useful revision techniques include reading aloud, either alone or to someone and who can test you on the topic. After reading over the material a couple of times, begin the note-making process. Mathematics is all about rules and principles. If you are able to deduce the important information and transcribe it as notes in your own words, you will find them these principles easier to apply to other areas.
Transfer your notes to flash cards. Remember to only include the most important information on the card; swamping your brain with information will not help it grasp the topic. Use your flashcards to test yourself by placing a card facedown and attempting to regurgitate its information without looking at it. Flashcards are an effective tool in rapidly learning a subject like mathematics. They are also extremely portable and will help you avoid carrying heavy textbooks and pads of paper on long bus journeys and car rides.
Memorize important bits of each principle. The point of the flashcards is similar to that of training wheels on a bike; they are there to wean you off the textbooks and make you more independent. When you have memorized the principles and facts on the flash cards, get rid of them. You can use your original notes as reference point and make more flashcards for topics you are less confident with.
Use your past exam papers and practice the questions repeatedly. GCSE courses are fundamentally a coaching program to ensure success in the final exam. It is vital that you know exactly what sort of questions will be on the exam and what sort of guise they will take. Your teacher or course tutor can provide you with past exam papers. Practice these papers until you are ready to sit the exam.