Decide how long you can realistially concentrate before needing a break. The recommended amount is 20 minutes, but for some people it's 25, 30, 40, 45 or an hour, so it varies. Then take a short brea; 5 minutes if you've only been revising for 20 or 25 minutes, 10 minutes if you've been revising for 30 or 40 minutes or 15 minutes if you've been revising for 45 minutes or an hour. Decide how many hours you can realistically do per day that leaves you enough time to fit everything else in and relax, but that will also be enough to get you the grades you're aiming for. Decide which subjects/topics, if any, you need to prioritise and whether you'd prefer to do one subject/topic per day or mix them up. Then make a timetable that takes into account all this information.