"Making/Writing a rough draft of a letter" means that you are outlining what you want to say in the letter, and that you expect it not to be your final work before sending the letter. You write down what you want to say in the letter, but you expect to read back through it and make changes before you send it. Hope that helps! :)

A rough draft is a first writing. It applies to essays and reports as well. You get the thoughts and ideas written down and then go back later to perfect the sentences and organization. What you submit or send in the end is the final draft.