For a Halloween mask, start by cutting out a strip of shiny paper that is slightly longer than the mask itself. (Alternatively, you could use some fabric or the frilly edge from a paper doily.)

Tape or glue the strip of material or paper on the back of the mask above the eyes, so that it sticks up above the mask with the shiny side pointing forward. Bunch the material up to make it fit.

Then make a hole at each side of the mask and thread the elastic through the holes. Make sure that the holes for the elastic aren't too near the edge. (Remember that you will first need to measure out the correct amount of elastic to fit your head!) Knot the ends securely.