Here's how I made it:
First, I used my (homemade) sleep mask as a template. It would be easy to sketch one if you don't already have a sleep mask, I'd just draw one big enough to comfortably cover the owner's eyes and then some.
Then I traced that onto the wrong side of my two fabrics. I like a soft, silky lining for the inside (the fabric that will be against the eyes.) I cut the two pieces about 1/2 inch larger all around and used my iron to press it along the traced line.
Then I cut out a double piece of cotton flannel about 1/4 inch smaller than the pressed pieces to sandwich between the two fancy fabrics. I pinned this 'sandwich' together, adding the elastic at the sides.
And then I put on some DVR'd 30 Rocks and sat down and hand-sewed all around the perimeter, making tiny stitches, and adding extra stitches around the elastic for strength. I think my sister could choose to wear it with either side facing out.