I've been making my own envelopes for a couple of years now and find that it makes mail both more fun to send and receive.
I've seen many a how-to on making ones own envelopes (you can find several on Instrucatbles.com), but none that seemed quite as easy and neat as what I do.
What you need:
A glue stick
Scrap paper (ie. calendar, scrapbook paper, magazine pages, etc.)
White paper or labels
Cardstock of some sort
An envelope of a shape you like.
1. Using a letter opener or knife, carefully open all sides of the envelope you would like to use. Lay it out flat and glue the edges with the glue stick. Turn it over and place it firmly onto the cardstock. Using your scissors, cut neatly around the edges of the envelope, which will leave you with an envelope, open, on cardstock. This is your envelope template. Make as many different sizes and shapes as your mind can conceive of.
2. Using your template, trace around it on the backside of whichever paper you will be using. Then fold it on the same folds that the original envelope used. Glue the bottom arch with your glue stick and fasten the bottom fold to it. Then fold over the top half, scoring it. As a finishing touch, add a large white label to the front for writing your recipients address. And you're done!
I was feeling a little crafty today, so I cut up some old 2009 calendars and made some Mr. Men and Butterfly envelopes: