Flannel (or other fabric of your choice)
Step 1: Take an existing pair of PJ pants that currently fit. Fold them in half. Fold your fabric into the center. Place the folded pants on the fabric, with the straight side lined up with the fold of the fabric. Pin in place.
Step 4: Take each leg piece and fold right sides together. Pin from the crotch down to the bottom. Sew a 1/2" seam where you just pinned. Repeat with other leg piece.
Step 9: Sew over where your inch gap was. And that's it! You're done.