Before you do anything regarding donation letters, please read this overview on Salsa CRM's word processing.
Here are videos that will walk you through the process of ensuring that you can quickly add your letters:
One of the most important aspects to generating donation letters is to make sure your letter contains variables so you don't have to type in the donor's name, address, donation amount, etc, every single time. There are different variables for donation, membership, acknowledgment, and summary letters within the system and "smart" variables that handle address blocks and salutations.
If you have documents you wish to edit that you have already uploaded to Salsa CRM, here is a great walk-through on how to add your variables to your letter.
Additionally, here are some best practices and letter templates that you can use as a guide for creating your letters or just edit them to make them your own.