how can i find and use avery template 5366 in libreoffice
Open Writer, in the menu click File | New |Labels . In the window that opens choose the Label tab, under Brand choose “Avery Letter Size”, Under Type select “5366 File Folder - White”.
For merge from database, select your merge database, table and fields. See Chapter 14, Mail Merge at https://wiki.documentfoundation.org/Documentation/Publications#LibreOffice_Writer_Guide
For a single repeated label enter your text in the “Label text” field, e.g. XYZ Corporation . Go to the Options tab and select “Synchronize contents” but keep “Entire Page” option checked. Note that with this option, if you leave the Label Text field blank you can type different text into each separate label box in the Writer document that is created. Cheers, Al