Summarize this article:

If you use a petite lace headband, you probably will not need to make any adjustments. If you use a standard-size lace headband, you might need to take it in.  You can use a white headband, but you can also try to find one in a color that matches your dress or your school colors. A petite lace headband is the sort of headband worn by babies and toddlers. To take the headband in, clip apart and remove the ribbon holding it together. Turn the headband inside-out and fold one end of the lace down until the entire length matches the length the garter needs to be. Line up the fold with the other end and stitch them together using a triple stitch or narrow zigzag stitch. Turn the garter right-side-out again. Cut 10 inches (25 cm) of narrow tulle. Fold it together and attach your embellishment with glue.  You should have about eight layers of folded tulle. Even at this thickness, the tulle will be light enough to prevent it from being seen at a distance. Put a small amount of hot glue in the middle of the tulle before firmly fixing your embellishment on the dot. Let dry. You could use fake pearls or gems for an embellishment, but for something more themed to homecoming, use a charm of your school mascot or school emblem. You could also attach a football jewelry charm or a small bow made with ribbon in your school colors. Hand stitch the tulle onto the center seam of the lace headband to complete your garter. Trim your tulle down before attaching it. Cut it near the embellishment, leaving just enough to sew into.

Summary:
Adjust a lace headband to fit around the wearer's thigh. Attach an embellishment to a thin piece of tulle. Stitch the tulle and embellishment onto the garter.