If you fully understand all the topics(especially the ones that are frequently tested in 6, and 7), then just do the past paper Q6, and Q7's. You have to be fully competent with the topic before you can master the 6 and 7. Sounds obvious but yeh.
The 6, and 7's usually test stuff a little but left field, that really take a little bit of abstract thought. Thus if you no the mechanics of a certain topic you can easily figure out what they want you to do.