Professor asked to write part of reference letter due to time constraints
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.