It's the same as preparing for any exam my friend, the key is to study - and study smart.
Cramming lots of information in all at once will put you into overload, but small and repetitive exercises is best for learning.
My suggestion would be to find a reliable source of training material, and dedicate a few hours each day to learning about this exam.
Penn Fosters themselves have released some information about studying for their exams and the transition they've made to a digital-only format:
I would also warn you to be wary of scams and tricks associated with this exam.
In my research, I discovered A LOT of fake sites offering answers to exams.
Don't follow these guys up. Not because it's not ethical to cheat, but because THEY DO NOT HAVE THE ANSWERS!
All they get you to do is fill out surveys or performing other actions for their website (like sharing on Facebook), and then they link you to a page full of advertising!
Best stick to the official training material, or other reputable sources.
And study hard!