Have you seen the AAMC guidelines for LORs?
The guidelines are intended to enhance the letter-writing process by providing a general framework of best practices and relevant content for letter writers.
students-residents.aamc.org
You should have been providing this to those you've been asking to write for you. I suppose you can use the guideline yourself and write as you'd like the writer to write (focus on stuff that person could say about you based on how you've performed in class, what they've observed about you (do you frequently contribute to class discussion, contribute less frequently but always synthesize what others have said bringing clarity to the topic area, etc).