Explain the problem. Take a look at this example complaint letter for ideas on how you should approach writing a letter of complaint.First of all, you should write your complaint in details, and in good language. If you wish to submit your complaint form via US mail, please complete, print and sign the Printable Complaint Form and submit it to: Be clear and concise. Describe the item or service you bought and the problem. Complaint Information. Please explain your complaint: Try to be brief, but be sure to tell WHAT happened, WHEN it happened and WHERE it happened. Consumer Action offers a sample complaint letter and an email complaint template. You can cut and paste these documents below.