This document explains the "Thank you" letter cycle and which tokens to use. In CiviCRM, "Thank you" letters that contain the substitution token for the specific contribution can only be printed from a search of contribution records. This means you need to follow a procedure of finding the "un-thanked" contributions and then doing an action of printing the "Thank you" letter. This is a different process then the one described elsewhere as "Printing Donor Acknowledgment" letters. The "Thank you" letter should be a Mailing template that you've prepared beforehand.
See the attached for detailed instructions.