The best advice i can give would be to study the rulings compendium on Pokegym. While it doesn't answer specific questions from the test (other than stuff like general rules), it contains rulings and game mechanics going back several years. I studied it well before taking the professor test and passed on my first try. It's also a good idea to look over the Play! Pokemon tournament rules, as some of the questions come from that as well, generally on judge calls and Spirit of the Game. Basically, just treat it like you would an exam in school.
The test structure is multiple choice and true/false. A sample question could look like this (I made this up, it's not from the test).
Keldeo-EX says that you may use its Abilitiy Rush in once during your turn. If you have two Keldeo-EX in play, how can you use the ability?
A. You may only use the ability of one Keldeo-EX during your turn.
B. You may use the ability twice; it doesn't matter which Pokemon uses it as you have two Keldeo in play
C. Each Keldeo-EX can use the abilitiy once for itself.
D. None of the above
If you've so much as glanced at a Keldeo-EX card, the answer should be obvious. On test questions that ask about rulings for specific cards, the exam will always give you the text of the ability or attack in question.