Hello. I just wrote one, and was accepted, so I'll tell you what I did...
I made everything relevant to the programme that I was applying to. I kind of wrote it as a simple essay. The introduction included my reasons/points for wanting to study the subject (relevant education/my qualifications/work experience). And then I expanded on those points in the body. Then wrote up a conclusion, which was something like, "you'll do really well in choosing me".
As far as style, I included a cover page. The university gave me a word limit. It was definitely no more than 500, but I can't remember exactly. Double spaced, times font.
Thanks, good luck to her!