Measure your model's head from ear to ear around the back, then subtract 1 inch. Now cut cut your elastic to this length.
Roughly cut a piece of the sparkly fabric bigger than the mask, and sew the felt piece onto it using a narrow seam allowance. Sewing with the felt on top will be the easiest.
Don't forget to put the elastic in between the two layers at the corner of the eye. Go ahead and back stitch over the elastic area to reinforce the elastic.
As you sew around the mask keep your elastic out of the way making sure not to sew over it when you do not intend to.
Your mask should now look like this from the front.
Carefully trim away the sparkly fabric making sure not to cut the elastic, and you are done!