CiviCRM will produce a list of contacts, with associated data, to use in a custom form letter.
First, use your favorite search method to generate the list of contacts (people) you want to send letters to. Contributions: Find Contributions is a good example.
With the results, from the "actions" drop-down menu, you can select "Export Contributions" or "Print or Email Contribution Receipts." The ""option lets you produce PDF receipts directly from CiviCRM. Use "Export Contributions" if you want the greater control you'll get using mail-merge in Microsoft Word.
" or "" depending on whether the primary fields give you all the data you want to include in your letter.
Here is a good tutorial for creating a form letter in Microsoft Word, and merging your exported data: