From experience, the best way is to first learn content, not too much but enough to give you a foundation and understanding of the subject and what each topic means. Then start a few past papers and add the answers from past papers to your notes. For maths it’s about technique and past papers really help from sites like A level maths revision, physics and maths tutor, mathspanda and YouTube is very helpful. For history, learn the content very well and then continue to repeat that every few days consistently for your memory then learn structure for each question that comes up then start past papers. And for product design I assume learning content then past papers is critical aswell. By the study period which is about May, you should be on past papers only and learning from them. Hope this helps!