This goes in the upper left hand corner of the front of the envelope. It should include a complete address so that the letter can be returned if necessary.
You do not have to type this in if you are using stationery preprinted with the return address. Stickers or rubber stamps may be used, although a sticker with a return address usually is reserved for personal letters.
If using an envelope with a double window, make sure the letter is folded and inserted so that the return address shows through the upper window.
With some personal letters written on stationery with a small envelope, it may be necessary to put the return address on the envelope flap after sealing the envelope.