Mark the slipper at the highest point of the arch, on both the left and right side of the slipper. To find the highest arch point, either put the shoe on or fold the heel of the shoe down flat against the inside of the shoe, marking where the fold starts at the top of the shoe.
Pin the first side of the elastic to the inside of the right side of the shoe. Take care to avoid the drawstring that circles the shoe.
Sew the right side piece of elastic to the slipper, again taking care to avoid the drawstring. Sewing through the drawstring will negate the drawstring's adjustment capabilities.
Put the shoe on. Pull the elastic across the foot to the other side of the slipper. Adjust the tightness of the elastic so it feels comfortable and mark or pin the point of attachment.
Sew the elastic at the marked point on the left side, again avoiding the drawstring.
Repeat for the second shoe. Test the fit of both shoes. Adjust if necessary.