Content of reference letter

I am applying for PhD and I need reference letter from my MsC professor. We made agreement that I write letter with my opinion and he will made changes later if needed.

I have no other "relationships" or "work" with professor except working on a project, actually research which was part of exam. I did research good, get good grade, but that is it. There was no lab, actually it was quantitative research based on interview.

Honestly, I don't know what to write.

I believe there should be description of project, communication with professor...

Any other suggestions?