Revise from the start. At GCSE you may have found starting to cram a few weeks prior to the exams worked fine. This is risky at Alevel. So, so risky. I am speaking from experience here. If you start reading over your notes regularly from the start, it will make revising easier, as you won't be trying to recall ALL THE INFORMATIONS at the last minute. Go through as many past papers as you can get your hands on; it's good practise so that the real exam won't be too much of a shock, and certain exam boards (speaking from experience with OCR and AQA) seem to recycle questions every so often (sometimes reworded slightly), so doing this could seriously pay off. Also make sure that you know the Assessment Objectives and Markscheme guidance backwards.
Whilst doing this, also switch up how you revise. It's all well and good spending entire days copying information from the text book, but good luck remembering it. You need to condense it in a way that is easier to remember and more familiar to you. I found that mind maps and flash cards helped rather than copying word for word. Talking to people about the information is a massive help too - perhaps debate a question with a family member or friend, chat with classmates, or even just teach your pet (that's what I did. My staffie was an expert in Russian history.)
This sounds stressful, but you only need to start with a little bit, and then do a little bit more as the exams get closer. You will feel more in control, and more able to cope with whatever they throw at you!