SDM = Strip Display Management
The test is basically a simulation of a control environment where you have to conduct a number of tasks such as updating flight progress strips. It's all explained very thoroughly before you begin the test and you get up to about an hour (if I remember correctly) to study the training material before you start. I really enjoyed this part of the day when I did mine, was good fun.
Basically you can't really prepare for stage 2, I believe that is part of the point - presumably allowing NATS too assess how quickly and efficiently you can pick up and learn new tasks, particularly whilst under pressure. Try not to worry about it and just be relaxed and focused on the day, everyone is in the same boat so you won't be the only one who is perhaps feeling a little under prepared.