Divide the body of the letter into 3 parts. You could have more paragraphs as well, but three is the minimum. Make sure the language that you use is decent and at the same time simple. This way the reader will not have any problem comprehending as to what you are trying to put across.
The first paragraph is usually the introduction paragraph. You introduce yourself, your firm and the purpose behind you writing the letter.
The second paragraph is where you elaborate on the purpose a little more. All that you want and wish to expect from the other end can be placed in this paragraph.
The third paragraph is where you thank the reader with pleasing words.