How to write complaint letters?

Firstly mention how do you know the person or company from which you have purchased the goods.
What is the problem in the order or goods.
What was the effect of the mistake on your firm or business.
What kind of solution do you now expect from them?
What if they do not do the needful? What do you plan next?

Keep up a polite tone in the letter. Do not be aggressive or blame them as it can be a mistake or just some misunderstanding while taking up the order. Having a polite attitude will encourage them to take necessary steps in order to solve the problem. The letter should contain all the details about the order, date of order, number of order, and every thing else. These details will help them to track back the order and they will be able to help you in a short span of time.