Cut silk organza to size for the petticoat, leaving the selvage edge to be the bottom hem. It should be taller than the skirt by about 4 inches. For width, multiply your flower girl’s hip measurement by 2.
Print multiple copies of the petticoat pattern on heat-transfer paper. Cut out design using a metal straightedge and craft knife on the straight lines and scissors on the scallops. Cut away any white at the ends of the strips.
Lay the iron-on pattern on the fabric aligned with the hem and flush with one end of the fabric, and iron on, following manufacturer’s instructions. Repeat with the other iron-ons, carefully matching designs at ends.
Finish sewing petticoat: Fold over fabric at top by 1/4 inch and press, then fold over by 1 inch, and stitch to create a channel. Fold fabric in half end-to-end, and sew together from hem to bottom of channel with a 1/2-inch or 5/8-inch seam allowance; do not sew the channel closed. Slip a long ribbon through the channel for a tie.