how do I insert a legal size envelope into my printer to be able to print the addressee ?
For inserting the envelope itself, that would depend on the printer itself. Might want to check your printer manufacture’s website for details on that. I’ve had different printers want it physically inserted in different manners.
As for formatting to print, this article on TechJunkie should help. https://www.techjunkie.com/how-to-print-an-envelope-with-libreoffice/