The envelope should be a standard size that matches the stationery (approximately 4"x9½" for standard 8½"x11" stationery). Fold the letter twice so that it is creased to make thirds. This will fit easily in a standard envelope, and it is easy to unfold.
The address of the recipient is in the middle of the envelope, beginning approximately halfway down. (Be sure it is mostly below the stamp, or it may get covered over by the cancellation.)
The return address is in the upper left hand corner. This is not necessary to type in if the stationery is preprinted with the return address.
If you are using business envelopes with a window, fold the letter so that the inside address shows through the window.
Use the block style letter if the envelope has a double window. This will make the return address appear in the upper window of the envelope.
Some correspondents include an attention line near the lower left corner for routing purposes. This is normally part of the main address unless space is a factor. It may be a department or a person's name.
Example: Attn: Returns Dept.
Due to variations in stationery size, it may be necessary to fold a personal letter differently to fit in the envelope that matches the stationery.
If the personal letter is in a small envelope, the return address may be written on the envelope flap after the envelope is sealed.
Click this link for Envelope Format Illustrated.
See also Folding a Standard Letter.