You first make sure you master the basics by doing from the 4U text; you attempt the questions yourself first and compare your solution with worked solution, or check with the worked solution after you have exhausted your unsuccessful attempts on the question.
After you have satisfied yourself you have a good grasp of the topics, you can then do past papers.
But, really, there are no hard and fast rules.You ca approach it any way you like, as long as it works for you.
