What is the best way to prepare for it?

There isn't much you can do to prepare the MLAT test in a short period before the exam. The MLAT is mainly a language test to make sure you have the basics. So what you want to do is buy a basic French grammar book with a recap of the basic grammar rules. Why not prepare a few flashcards flashcards on things like the COD, the COI, the passé composé and so on. In terms of vocabulary, there isn't much you can do since the sentences can be about absolutely anything. Although doing some French reading beforehand could help to get you into the logic of French syntax and wording.