Go to file and and click on the paper size option. You should be able to find the required envelope size in the drop-down menu. If I can do it anybody can.
My HP printer has a special slot for envelopes, I just feed them into this.
If you are printing your letter in Microsoft Word, click tools on top menu, select envelopes and labels, in options it will give you the chance to select the size of the envelope.
I always choose DL.after I have printed the address, I feed my envelope into my printer and select Print.
type the address on a Word page; hilite the address & press Ctrl + c to copy. Open Tools/letters/envelopes & labels. Press Ctrl + v to paste address and print to envelope inserted into printer - as instructed and print.
If you only want to print less than then number of labels on the sheet you risk not being able to reuse that sheet. I would not bother trying to reuse a "sheet" that I had detached some labels from already. The printer tends to not like it and has a nasty habit of mangling the sheet.