I simply went on Google and searched for "vintage lace background," found the one I liked, opened it up in Publisher to the size I wanted it (just slightly more narrow than the envelopes) and printed them off. Then, using a paper cutter, I cut them down to size.
My next step was to glue my paper and then just slip it in the envelope, pressing it down to make it stick.
The only problem? Slapping pretty paper on the inside doesn't help the envelope look pretty upon arrival. So I took my same lace image, opened it up in Publisher, cropped it down to the sizes I needed for return labels and address labels, then typed up the addresses in both. I printed them off, cut them out and glued them on with a glue stick.
Seriously - easiest upgrade ever. Best part? It was free. Between designing these invites myself and fixing up some envelopes I already had, I managed to spend less than $7 on a great first party impression. Hope our friends and family enjoy opening these up!