Use the chosen lace as a measuring tool. Align the deepest part of the scallop of the lace (called the LoPoL) along the bottom hem line. If the lace is wide enough to fit pieces A and B, add the seam allowance to the edges between A and B, and you are done.
Sew A to C by overlapping the lace and zig-zagging them together. Then sew the A/C unit to B. Repeat for the other side. Sew the centre front and centre back seams, then sew the inseam. Apply elastic to the waistline. The boy shorts made from stretch lace are done. There is no need for elastic along the leg openings.