How do I prepare for unseens in the exam?

I would prioritise your principal parts learning; make an effort to go through, say, 10 a day on flashcards and break it down - it seems like a big task but it can easily be made simple.
Go online also and get your hands on past papers; to start, give them a go and highlight the text where you get stuck then go into your notes and find the answers or vocab. As you go along make a list of vocabulary you're not familiar with (I reccomend having a little book to do this in).
Eventually you will find yourself less and less reliant on notes as you come across more structures and sentence types.