Cal PatchGrabbed a pair that fits well, an old partial T, and some quarter-inch elastic, and here's what I did:
1. Cut out the pieces. Fold your underwear in half along the center front, carefully lining up the side seams and leg openings. Lay it on top of the folded T-shirt, aligning the folds.
Try to get the whole front piece to lie flat, but you may only be able to work with one section at a time. Cut around the edges, adding 1/4-inch seam allowance as you go. Only cut around the parts that lie flat, then flatten the rest and finish cutting.
Repeat for the back.
Lay the cut-out front piece on another folded scrap of shirt and cut around the crotch area as shown, for the separate crotch panel found in most undies.
2. Sew the seams. With right sides together and the front piece on the bottom, line up the back and front along the crotch seam. Lay the panel piece on top, face down.
Seam all three layers together at 1/4 inch. Flip the panel up so it lies against the front and the seam is enclosed inside, and pin it in place along the leg opening edges. (The front edge of the panel usually remains unseamed, but you can stitch it down now if you want. I used the hem of the T-shirt for this edge so it was already finished, but you could hem or serge it, or leave it raw.) Now put the front and back together, with right sides facing, and pin and sew the side seams.
Turn the underwear right-side out.
3. Sew the elastic edges. Starting at one side seam, lay an end of elastic on the right side (seems wrong I know, but trust me here!) edge of the waistline, matching the edge of the elastic to the waist edge. Begin stitching (I used a three-step zigzag, but a regular zigzag works, too), stretching the elastic ever so slightly as you go.
You don't want it to gather the edge, but you do want a snug fit. Work around the entire waistline, overlapping the elastic 1/2 inch where you began, and cut off the excess. Be sure to backstitch securely.
Then flip the elastic down onto the wrong side (inside) of the pants and stitch again, this time working from the wrong side (elastic side up).
Repeat the process for each of the leg openings, enclosing the panel into the seams as you go.
And they're done!