Short answer: Use previous year questions chapter by chapter while learning, then attempt full papers under timed conditions, and keep a log of every mistake.
Start chapter-wise
After finishing a chapter, solve its previous year questions straight away. This shows you what the exam actually asks and where your understanding is thin, while the chapter is still fresh.
Move to full papers later
Once most of the syllabus is covered, attempt full papers in one sitting with a timer. Practise skipping hard questions and returning to them, since managing time is a skill of its own.
Keep an error log
Write down each wrong or skipped question with the reason: concept gap, calculation slip or time pressure. Reviewing this log weekly is more valuable than attempting yet another paper.
New or used papers
If you buy a used paper book, check whether the answers or attempts are already marked in pencil. A clean copy matters for papers you plan to attempt under exam conditions, while a used copy is fine for reading solutions.
Frequently asked questions
When should I start solving previous year papers?
Start chapter-wise questions as soon as you finish each chapter, and begin full timed papers once most of the syllabus is done.
How many papers should I solve?
There is no magic number. Quality of review matters more, so analyse each attempt before starting the next one.
Is it okay to buy a used paper book?
Yes if the pages are unmarked or you only need the solutions. For timed practice, prefer a clean copy.