Legal writing is probably the worst course to prep for. If it's graded, your grade is going to be based on matching what you do with that the instructor expects. So even if you learn a great way of doing things, if it's not the instructor's way, it can lead to a poor grade. And if it isn't graded and you're that committed to doing prep work, doing it for something that's graded.
The grammar, structure, and spelling advice is best. There's a ton of law students who are starting out with high school or lower writing skills. If you're a college level writer, you can rest easy. The legal writing skills will come.