There are different types of copyright letter. Two common types are:
- a permission letter, under which one person grants another the right to use a copyright work; and
- a letter before action / letter of claim / cease and desist letter, in which the copyright owner demands that another person stop infringing copyright.
There are subtleties to both types of letter, so if it's feasible you should ask a copyright lawyer to write the letter.
The next best option would be to use a template, for example: