I had to write myself an LOR because my boss was too lazy to do so. (I didn't use it for med school apps, though - I used it for job searching). Ironically, interviewers who received the recommendation questioned me about how close my relationship was with my boss, because they said the letter seemed "too impersonal". It was pretty hilarious. In retrospect, I think I wrote myself such a bland letter because I felt awkward and afraid about writing myself an LOR (I personally wouldn't do it again, and wouldn't have done it in this case had I not been forced to). I didn't want to be caught.
So, I'm not sure I'm a good role model for this question, but I wrote that I was a diligent, intelligent, capable person who could be trusted to complete tasks efficiently and accurately, and that I was also a pleasant person to work with. I also described my duties and projects. The letter was about 250 words long.
I suppose if you want to avoid falling into my trap, sprinkle your letter with one or two anecdotes about how you went beyond the call of duty or exhibited positive personality traits. I hope this helps.