Fold And Go Changing Mat Proper Way To Fold An Enclosure Envelope Inside Of Another Envelope Without Changing Envelope Sizes?

Proper way to fold an enclosure envelope inside of another envelope without changing envelope sizes? - fold and go changing mat

Is there a proper way to fold a sealed envelope in another envelope without the envelope size? Folded in half the length, width-wise, or neighborhood? What is the accepted method for sending a professional letter?