I was lucky enough to have access to letterpress equipment, but any stamp, stencil, screenprint, etc. method would work. I chose a porous cardstock because the ink dries quickly and you can keep working. Once I figured out my card dimensions and registration, I printed the font and back on one side (unfolded - you do that last).
Step 2: Inside
Next I printed the inside text. Again, figuring out registration is the tricky part. For those who have never been to Chicago, we call our train, "the el" (as in "elevated"). I'm only giving this to fellow Chicagoans so the lame inside joke will hopefully not be lost. Then I used a bone folder to crease each side down the middle for easy folding. I had a friend take a snapshot of myself waiting for the "el" and pasted the prints inside.
Step 3: Envelope
I found some matching kraft paper envelopes so as a finishing touch I added "merry christmas" in gold ink