Everybody's worst topic. There is no set way of doing these after you have the basics and understand the factorials so it comes down to practice and finding a way to link each question. There are plenty of tutorials out there to explain this; try the below.
All the youtube channels below have a series of videos and are useful for all Statistics topics in AS. They also have pdf/docs of problems and answers so use these.
Good luck
For the basics.
To help you learn what the topic is and the very basics. Lesson 1 Lesson 2
For the harder exam-style problems
Option 1 Mathlete. He has a series of videos here that cover all the different types of problems - Perms and Combs
Option 2 Exam hacks. A well-explained set of tutorials, but they don't cover all problems. Exam hacks