Bring the flap of fabric up and around the rolled-up briefs body so that it matches up with the pinned part. Removing one pin at a time, add this flap to the other two pieces of fabric, with the right side facing the other pieces.
Back view of pinned sections.
Front view of pinned sections.
Using an overlock stitch, a slightly longer than normal straight stitch finished with a zigzag stich, or using a serger, sew and finish off the edges of the crotch seam.
Turn right side out and pin the lower edges of the fly stays to the darts on the front of the brief body.
Starting at the pins, baste the edges of the crotch pieces together. This will make applying the leg bands easier.
A pic of the basted crotch area.
Take the leg bands and, right sides together, sew the short ends together with a straight stitch.
Fold the pieces wrong sides together, lengthwise, and mark the center opposite the seam. Mark the center opposite the crotch seam on the brief body. Match up the seams and centers and pin. Stretching the leg band to fit, pin in a couple places between the other pins so that it is evenly distributed. Sew bands to brief body using a medium zigzag stitch.
Fold the bands over the seams and under them, then pin to make a narrow leg facing.
Using the medium zigzag stitch, sew around the top side of the binding close to the seam.
Picture of the leg bands applied to the brief body
Cut elastic as directed in your pattern. Note: My pattern calls for 1 1/4" elastic, but since my grandsons wear a size 6, I used a 1" elastic instead. I've also used FOE. It really doesn't matter as long as it looks nice.
Using a straight stitch, sew the elastic right sides together at the short ends. On most waistband elastics, one side is a bit softer than the other; this is how you can tell which side is the right side.
Finger-press the seam open and sew it open using a lightning zigzag stitch.
At this point I applied the top only of the size tag over the center back seam.
Using pins, quarter and mark the elastic and the brief body. Slide the elastic over the briefs and line up the pins and attach the elastic to the briefs, placing the wrong side of the elastic against the right side of the brief body.
Sew the elastic to the brief body, using a lightning zigzag stitch and stretching the elastic as needed.
Trim the fabric under the elastic, being careful not to cut through the elastic or briefs fabric. At this point I attached the bottom of the size tag with the lightning zigzag stitch so it conceals the center seam; this helps prevent irritation and itchiness.
A picture of the finished briefs.
A picture of the 3 pair of briefs I made this weekend for my daughter's 3 potty-trained boys.