There is a tutorial
here that uses the Crafter's Companion tool.
Or if you have a Scor-Pal, here is a
tutorial on their site.
If you have neither, you could do this. Open all the seams of an envelope of the correct size. Lay it on a manila folder or a Cheerios box. Trace and cut out a pattern. Then you can trace around that pattern onto any paper to make your own envelopes.