How do I print a A9 emvelope?
Do you real mean ISO A9 size, i.e. 37 x 52 mm, which is hardly the size of a label?
Printing any type/size envelope is fairly simple in Writer. From the menu select
Insert->Envelope (at bottom of Insert). Since the A9 format is not in the selections, go to the Format tab and there you can insert the dimensions - 5 3/4" x 8 3/4". You can also declare where the addresses are to be positioned. On the Envelope tab you can enter the address details and they can also be entered/modified on the document once you completed the dialog.