Stitch down 1/4” from the top and 1/4” from the bottom of the casing. This will hold the ribbons in place and create a space for the elastic to fit through. Leave a 2” opening at the end to push the elastic through. Measure the circumference of the child’s chest. Take two inches off this measurement to ensure a good fit and cut the elastic to that size. Attach a large safety pin to one end of the elastic and thread it through the casing.
When the elastic has made it all the way around, stitch the two ends together.