Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear Mr. ____ and Mr. ______,
I'm writing in regards to the quality of service/food I received at your restaurant at [address].
I've been a diner at your restaurant for some time, and have come to expect a high-quality dining experience. The incident on [date] left me wondering if your restaurant is in decline or whether this was an unfortunate, one-time incident.
My complaint, specifically, is as follows: [describe nature of complaint]
I look forward to your response. You can reach me at [email address] or by snail mail at: