How to resign/change status gracefully?

So recently I was offered a job in a progressive care unit and I accepted it. I'm still waiting back on a local hospital OR position but I'm under the assumption I'm going to start in the PCU on the 5th. Currently I'm working nights and I gave verbal notice, my supervisor told me when I come back tomorrow to give her a written notice. How exactly is the format for such things?
Google "sample resignation letters" and you will find dozens of examples of what to say and the format in which to say it. It's really not all that hard, especially since you have already given verbal notice. The hospital just wants a written record of your resignation in their files in case you would ever try to come back on them and say that they fired you unjustly or some such thing. Sounds dumb, but it happens, and they have to protect themselves.