My ideal way to prepare...

at the beginning, I wrote some drafts, but, none of them was used. all referenees wrote letters by themselve.

Just try your best, and list some important items instead of a whole letter.

ideal way: give referenees a list of things he/she need to mention and several good letters from your lawyer or friends. American guys know how to write better letters than us. This is eactly what I did.