I usually take the patterned paper from stacks that I don't like (white on opposite side) and make envelopes (pattern to the inside) so that it makes a nice lined envelope. The other way is to take an envelope and use it as a template, and then trim off about 1/4" around and about 3/8 - 1/2" on the top of the flap so it doesn't cover the gummed line. You don't need to do the entire envelope, just the front and flap. Basically, the purpose of a liner (in the old days) was so you couldn't see through the envelope. People are pretty nosey and would hold the envelope up to a light to see what was inside. I use to do this when mail came from my school - a looooonggg time ago.
Lining envelopes
