https://www.letterstrellis.com/activities/origami-letters
Folding a letter size sheet (8-1/2 x 11 inch) into its own envelope for mailing. The above page has a video instruction, and a template to follow for the folds.
There is a QR code that can be stuck to the back of the envelope to assist the recipient how to open the letter without damaging it.
Finally, some information on "letterlocking" - a number of methods, including some fancy folding and tucking (and cutting) that were used to assure that a letter had not been tampered with in transit. Mary Queen of Scots locked her last letter using some of these techniques before she was beheaded.
