This is painful. Consider using Microsoft Office or switching careers. If you need to write < 20 letters consider doing it manually.

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So you didn't listen and here it comes:

1. Ignore the Mail Merge Wizard. It will crash or destroy your document.
2. Export your addresses, recipient names, etc. as a .ods spreadsheet (.xls, .xlsx, .ods). Use any columns that work for you, but be consistent. I like to use one column for the address, one column for the salutation line.
3. Import the spreadsheet as an address book source: Tools => Address Book Source => Address Data Source... => Other external data source => Settings => Choose "Spreadsheet" as Database type => Next => Pick spreadsheet file => Ignore field assignments => Give it a name
4. Back in your document you can now insert placeholders for a column value. Use menu Insert => Field => More fields... => Mail Merge Fields. Select the address book database you imported earlier. Pick the column you want. Insert.
5. Print the document with File => Print. LibreOffice will ask you if you would like to print a form letter.

If Writer keeps crashing on the last step, recreate your letter template from a blank page like a wild animal. Be careful to paste text without formatting or styles.