The postage (stamp, meter imprint, or prepaid imprint) goes in the upper right hand corner of the envelope.
Do not cover a stamp with adhesive tape.
Make sure the stamp is positioned so that its cancellation will not affect the addresses.
Be sure there is enough postage. Always check letters of more than three pages, letters with inserts, or letters to foreign countries for correct postage. There are few things that will annoy a correspondent more than having to pick up and pay for a postage due letter.