Elastic enough to stretch across your backfor me this was 10 inches
Make the sash. Start by cutting or tearing your fabric into 3 pieces each 6×44 inches long. Then cut one of those pieces down to the the length of the distance of the front of your waist (basically from one side to the other for me this is 17 inches).
Next sew your your short edges of one of your long pieces to the short edge of your middle piece right sides together.
Repeat with the other short edge of the middle piece and the other piece of fabric right sides together. Now you should have a long strip of fabric.
Next fold the sash in half right sides together. Sew down an angle at the short edge and then down the length of the fabric with a 1/4 inch seam allowance. Leave the other short edge open for easy turning.
Turn your sash right side out. I like using a loop turner for this.
Iron your sash making sure to tuck the open end in at an angle.
Sew across the end to close the hole up. Set your sash piece aside.
Sew your your skirt by sewing the side edges of the skirt pieces with right sides together.
Finish the top and bottom edges of the skirt with a serger, zig zag stitch or pinking shears.