Honestly, understanding and knowing how to use the contrapositive is one of the most crucial techniques for the entire test. It comes up very often and is critical to answering many questions. It is also extremely important in logic games.
To get a contrapositive. Simply reverse and negate a conditional statement.
A -> B
The contrapositive would reverse the two variables and then negate them.
NOT B -> NOT A
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further!