Fill jug or deep mug with 4 parts boiling water to 1 part cold water. (Deep enough to dip 3/4 of the arms of the eyewear into the water).

Dip both sunglass arms 3/4 way into the water and hold for 15 seconds.

Take out and bend sunglasses to shape.

(for a looser fit, pull arms away from one another - hold for 15 seconds).
(for a tighter fit, bend arms towards one another - hold for 15 seconds).

Sit them down on a table to ensure both tips touch the surface. This will ensure the frame is not twisted.

The end tips of the frames can also be adjusted as per above if they curve too sharply behind the head or if they don't curve enough.

This can be repeated until desired fit is achieved.