Cut a 13-by-118-inch rectangle of fabric (you may need to link together several 13-inch-wide strips with fabric glue, overlapping ends, to create 1 long strip). Coat with fabric stiffener, using a foam brush to distribute evenly. Air-dry for 1 hour. Press with an iron.

Lay an 11 1/2-foot length of grosgrain ribbon on each long side of skirt (see top), positioning ribbon so that there's 10 inches of overhang on both sides (these ends will form ties). Starting at 1 edge, affix each ribbon with fabric glue; stop 2 inches from opposite edge, and leave remaining ribbon loose. Repeat with a third 11 1/2-foot length of ribbon (see top), positioning it 2 inches from 1 side.

Using a quilting ruler as a guide, make 2 1/2-inch-wide accordion folds in fabric (last fold will be slightly narrower). Press fabric, and unfold. 

Working along the side with 1 ribbon, punch holes through ribbon and fabric at each fold, about 1/4 inch from crease. 

Thread 1 yard of cord through holes. Wrap skirt around tree, tying cord and ribbons at back.