Professor asked to write part of reference letter due to time constraints I am applying to graduate programs and one of my research professors will write me a recommendation letter, but wants me to write a rough outline of the letter. He will add his own content and edit all my work, but wants me to highlight things he might not know.

I am not sure how to write this outline, advice from people who have written graduate recommendations on what to put in one?
This is pretty common, actually. 

I'd suggest making it an outline, not a narrative. But use complete sentences so that he can copy paste if he wishes. Include the things from your background that you want highlighted in a letter. But, I'd probably not recommend writing glowing praise for yourself. Let him add that part. 

You can also include some forward looking things, such as what you intend to focus on in grad school.