I spent about 6 weeks doing flight planning, and I self studied with AFTs notes.
The trick to the exam is understanding the concept and what you are trying to achieve.
It helps to be practical about it too.
The biggest tip I can give folk is if you get a result that is between two answers, always pick the lowest fuel burn figure as an answer.
The more sectors you break your plan into, the more fuel burns you can calculate and the lower the burn in each sector as the aircraft gets lighter.
If you understand this, you will pass flight planning first go with at least an 80% pass mark.