Here's an example fired off the hip.
On official letterhead
Dear MSTP Selection Committee,
I am writing to offer my enthusiastic support for Madindalab for the Medical Scientist Training Program of the University of X. I have known Madindalab since he/she joined our group as a research assistant in Month of Year. Since that time he has worked part and full-time on a project entitled "The random interaction of protein X and protein Y: implications for a disease you've never heard of."
As a research assistant, Madindalab has made exceptional progress. He took the lead in interpreting the basic research behind this project, designing experiments pertaining to the hypothesis, and in interpretation of the data and future directions for the project. [insert personal anecdote here: dedication (once I found out he worked all night for a deadline without any prodding), charity (he dedicates his time selflessly to the training of new students), knowledge, ability, something]. His hard work has culminated in a poster presented at the International Conference of Disease You've Never Heard Of, and a manuscript in preparation (if true).
I have supervised many students over the years, and Madindalab has consistently impressed me in his medical and research knowledge, dedication, and ability. I have supervised many students in research over my career, and Madindalab is simply outstanding. He has told me that his goal is to become a physician and scientist. Based on my interactions with him, I believe that he would be exceptional in that role. Madindalab should become a member in the Medical Scientist Training Program so that he may develop his knowledge and skills to become an excellent physician and scientist.
Sincerely,
Not writing about myself