Let me start by saying I didn't prewash my fabric, I didn't plan, I didn't research and I didn't use heavy-duty thread, but they turned out great and cost under $30.
Supplies: $10.50 Forever 21 stretchy skinny jeans
$16.99 Belly Band from Target
I started by cutting out the zipper and stitching the fly closed. (My sewing machine isn't heavy-duty enough to handle sewing over a zipper).
After sewing the fly shut, cut around the front. I cut lower in front to accommodate the belly.
Put right sides together...
…and stitch along the seam of the belly band.
Flip them right-side out and try them on!