There are some great posts in this forum about how to prepare for law school during 0L, but it seems they mostly discuss prep for the substantive courses.
Any advice on how to prepare for Legal Writing class during 0L?
I honestly wouldn't say there's much/anything you can do to prepare for Legal Writing. At least there's not anything that's time-effective. You'll understand when you take the course, but anybody who claims they can make you a better writer either:
1. Is full of crap.
2. Presumes you write at the level of a mental deficient and all they're going to do is make you a slightly less obvious mental deficient.
Legal writing is mostly about just getting in there, doing the research, writing, then tightening, tightening, tightening. You can't do much to prepare for it.
Contrast to your other subjects where, frankly, you can learn 80% of the entire subject prior to your first day in class.