I know a lot of people who've made their own invitations, but I do think mine turned out well, so I thought I'd share.
The invitations were quarter-sheet sized (8.5x11"), because envelopes were available for that size, and it was an efficient use of paper. I used parchment-look envelopes.
The invitation itself was a piece of textured heavy watercolor paper, stamped randomly with leaves in a few shades of green.
The wording was printed on vellum in green ink. We used a simple calligraphic font (BibleScript, I think it's called). The vellum was cut about an inch and a half smaller each way than the watercolor paper and centered on the paper. Then I punched two holes in the top of the vellum (through both layers) with a small hole punch, and tied the two together with a narrow green ribbon.
Two of my friends joined my mom and me to assemble the invitations, and it went quite quickly.
My 12-year-old niece was into cardmaking at that time (she's now doing a lot of chainmaille), and so I had her do the stamping for me, as well as making thank-you cards with the same stamps and stamp pads. I think I still have a few around here somewhere...