What is your proposal?
Basically, think of the 5W's and 1H as in Who, What, When, Where, and Why. Who are you writing to? What is this proposal about? What are you willing to do? Ask yourself these types of questions.
Then at the end of the letter, just like a conclusion on a research paper, sum it all up in one or two sentences.
At the very end, make sure you close your letter with Sincerely,