Use the standard business letter format. If you have forgotten how to format a business letter, you can use any of the free templates available online. A business letter should have the name and address of the person you are writing to at the top of the page. Also, include the date. If there is a reference or order number, indicate that just above the greeting.
Begin with a sentence or two complimenting the contractor on something he did well. For example, you could write: "I appreciate the time you took to help me pick out the right color paint ...." This will help soften the blow of your complaint.
Stick to the facts of your situation. Be specific in your complaint and how you want to rectify this matter. For instance, "Unfortunately, when you came to paint my living room on May 1, 20xx, you did not prep the room properly and got paint all over the light fixtures. I would like you to pay for the damages and purchase me a new light fixture for the overhead light in the living room. I have priced similar fixtures at my hardware store and they cost about $100. Please send me a check for this amount or if you can buy the fixture wholesale, please do so."
Close your letter in a way that indicates you expect the contractor to deal with the issue by a particular date. If this is the first time you are addressing the problem, you can end by giving the contractor the benefit of the doubt, such as: "I'm sure you didn't intend for this to happen and that you will take care of it by May 30, 20xx. Thank you for your quick attention to this problem."
Remain professional and businesslike throughout your letter. Do not swear, use emoticons, excessive capitalization or exclamation points. State your problem calmly and succinctly and how you would like it to be resolved. Be sure to proofread and spell-check the letter before sending.
Include copies of receipts, photos or other documentation in your letter. Do not send the original documents.
Mailing the letter via certified mail requires a signature and is for your own protection, should you need to take the contractor to court. The contractor would not be able to argue that he never received the letter when you have a receipt with his signature on it.