Followed these instructions to make DIY envelope liners: a simple and low cost way to decorate my otherwise cheap and plain “$1-for-20” white envelopes.  Had extra pattern papers for lining the smaller brown ones too!

And because I used budget envelopes (paper is of the thinnest quality, not Conqueror type) for this project, I could not disassemble the envelopes without tearing them up.  I overcome this problem by simply outlining the envelope with its flap up on the pattern paper, cut this out, and then trim 1cm away only around the flap border.  After that, Insert the liner into the envelope and apply glue on the underside of the pattern paper to secure.  It might not be as neat as if you were to following the original instructions, but hey, no one is going to disassemble the envelopes just to check either!

I find simple monotone pattern papers work best for this project.  You can print your own envelope patterns like these, or browse around here and here to download free paper patterns for personal use.