A scrap of soft fabric--think sherpa, fleece, etc (a 10 by 10 square was plenty for our project)
Sew on velcro
Scissors, matching thread, tape measure, etc
1. Measure the width of the seat belt. Multiply it by two (so the cover can wrap around the belt. Measure the width of the velcro and multiply by two, adding it to the total width of the fabric. Add about an additional inch for seam allowance.
2. Measure how long you'd like your seat belt cover. Add about one inch for seam allowance. Cut out fabric.