Topical past papers sort real exam questions by syllabus topic, so you can practise one topic at a time instead of waiting to sit a whole paper. Here is the method that gets the most out of them.
Read your notes or textbook section on one topic, for example Forces or Quadratics. Do not try to memorise everything. Ten to twenty minutes is enough, because the questions will show you what you do not yet know.
Take the first group of questions on that topic and answer them on paper, with the mark scheme closed. Leave a blank where you are stuck rather than guessing and moving on. Writing the full answer matters: Cambridge awards marks for method and for the exact words in the mark scheme, not only for the final answer.
Mark every answer with the mark scheme beside you. Be strict. If the scheme wants a particular word and you wrote something vaguer, that is a lost mark, and it will be a lost mark in the exam too. Where examiner comments are printed with a question, read them: they tell you what candidates commonly got wrong.
For every mark lost, write one line: the topic, what you did wrong, and what the right idea is. Sort the reasons into three kinds: did not know it, knew it but made a slip, and misread the question. Each needs a different fix. The first needs revision, the second needs care, and the third needs the habit of underlining the command word and the numbers.
Return to the questions you got wrong after seven days and try them again cold. If you can do them now, the topic is in. If not, go back to step 1 for that topic only.
Once a topic feels secure, do its questions against time. A rough guide is about one minute per mark. Finish with full papers under exam conditions in the weeks before the exam.
Many students find it easier to work from a printed question bank with room to write the answer on the page, because it keeps the questions, your working and the mark scheme in one place. ExamYard publishes printed topical question books for IGCSE, O Level and A Level, and the first chapter of every book is free to read online.