The bad news is that Mail Merge in LibreOffice sucks even worse than it does in Word. The good news is that someone has figured out how to actually make it work with the minimum of pain.
Rule number 1: Do not use the Mail Merge Wizard, at least not until you’ve already manually mapped the fields of your Calc data sheet to where you want the data to appear in your Writer document. The Wizard is hopelessly broken, do not rely on it. At all.
Rule number 2: If you’re carrying over a merge document and data from Word and Excel first convert both to ODF format. Then remove the mappings from the Writer document before going any further.
Now read this article. Then read it again.
Basically you’ll be linking your spreadsheet as a data source for your document, then dragging and dropping the appropriate columns from the spreadsheet where you want them to show up in the document.
1. Open your document in Writer.
2. Go to Edit… Exchange Database. Browse and select your spreadsheet.
3. Go to View… Data Sources. Pick your spreadsheet in the left hand window. The spreadsheet data will be in the right hand window.
4. Select each column from the spreadsheet you want to plug into the document, and then drag and drop into place.
5. Click Tools… Mail Merge Wizard. Step through and uncheck all the options.