I was in the enviable position of writing several...A few things to keep in mind (others may disagree):
1) keep it short and to the point - a paragraph will do, they do not want all the lurid details (but don't be so vague that it allows them to imagine the worst)
2) DO NOT come across as whiny (sounds obvious but I would have several people read yours just to make sure it doesn't sound that way to other people- when it's your own writing I think that's easy to miss)
3) offer to provide additional information or any relevant documentation
4) take responsibility for your actions/decisions
5) if there was some sort of lesson you should have learned, make it clear that you did
Good luck
