Case studies are different for each school, as it doesn't specifically say in the markscheme which ones to use. So use the ones your teacher told you to, cause they'll make most sense to you.
To revise the casestudies I just used lots of colour, and made little case study cards that I carried around in my pocket to read at bus stops etc.
And the rest of it really is common sense. As long as you can read a map and empathise with the harsh conditions other people live in around the world. I got an A* last year on the WJEC course.