How to write a letter of agreement between two parties examples and how to write: A consent letter is used to define the relationship between the parties to meet certain expectations. This letter can be used as a legal document when signing contracts, so you should be aware of everything you put in your contract letter. Here are some tips you pass forward; The consent letter can be a basic agreement on the default letterhead, written by one of the parties. It is the party that makes the offer that most often writes the letter. In this way, you can also control the terms of the agreement, provided that the general understanding is reflected. The letter must therefore specify when the agreement will enter into force and when it will end.