With right sides together, stitch seam 22. This is the back half of the crotch. You should now have a tube that doesn’t look like boxers at all.
Fold so that the seams 25 are on the each side and the bottom points of seams 22 and 23 meet in the middle to form the crotch.
Stitch the end of the casing closed. Be sure the end of the elastic is stitched in place.
Line up the hems of both legs and sew seam 26 from hem to hem.