Be polite and reasonable in your letter. The person who will read it probably didn't cause the problem, but may be very helpful in resolving it.Download and complete the Attorney General's Office Complaint Form. The following is a brief outline for the structure of a complaint letter about a purchased product. Describe the item or service you bought and the problem. Include serial or model numbers, and the name and location of the seller. You write them a letter explaining what happened and why you have a complaint. Look up the corporate address online. File a complaint against a business with BBB, search for a business to file a complaint against, or find out the status of a complaint you submitted. File your complaint anonymously.