Prepare the list
One sheet. Row 1 is headers: FirstName, LastName, Address1, City, Postcode. No merged cells in that row. No blank rows in the middle. Keep it as XLSX. If you must use CSV, save UTF-8, especially for Arabic, Russian, or CJK names. Excel “CSV ANSI” will mojibake them in Writer.
Insert fields
In Writer, start the mail merge wizard or Mailings-style commands (names vary by build). Insert fields where the name and address should appear. Use a real paragraph style, not a text box, so line spacing survives.
Test three rows
Preview records 1, 2, and the last row. Long Polish names and two-line Brazilian addresses are where layouts burst. Then merge to a new document, not straight to the printer. PDF that file if you are sending a mail house a digital pack.
Labels
Match the Avery (or local) product code. Printers scale. Print one sheet of cheap paper, hold it over the label stock against a window, then commit. If the suite’s label wizard is Premium-locked in your build, a table with fixed cell sizes in Writer is the unglamorous workaround.
When to stay in Word
If the office already has a Word data source with Ask/Fill-in fields or a complex filter, do not translate it the night of the gala. Merge in Word, or export a clean XLSX and rebuild. Ego is cheaper than 400 wrong envelopes. Writer chapter.