You know how to write... use the examples you find as a guide, but it really only needs to be a few sentences letting them know when you are leaving, and a pleasantry or two so they know it's friendly.
Writing the letter won't be horribly hard, I think. It's following the rules that makes me nervous! And I wonder who it will go to after I show it to my boss and what will happen and if there will be hard feelings from some in management or what.
I know, I know...I worry too much.
Just copy Hair's letter, inserting your own personal info, of course. ;o) It was really good!