First, I've searched for a pattern. You can do one yourself, either using an old sleeping mask as template or just sketch it by hand (be sure, that your pattern covers your eyes properly and that it is symmetrical!). Or you can search the internet for a free pattern

In the next step, you want to cut your three layers, using the pattern (front and back of your sleeping mask, as well as the interfacing) and some ribbon, or, in my case, elastic in a matching color

put your layers together in the following way:

right sides of your bedcloth fabric facing and on top of that, the interfacing

Make sure (especially if you have pictures or writing) that your layers match and insert the ribbon/elastic.
Try to flatten it as much as possible and be carefull where to put elastic-ends. If you would fold your mask over in the middle, the ends of the ribbons should met.

pin everything together and make sure that your elastic stays in place. Moreover, you want to leave a little hole to turn your mask inside out after sewing (the space between my fingers)

sew

make sure that your pet does not interfere ;-)

When sewn, you can take the hole...

...and turn your mask inside out

Topstitch it near the edges and press.

With any luck, you now have a dazzling sleeping mask ^^

Don't worry if your first version looks poor... my first attempt looks like a troll made it -.-, but once you've got the hang of it, it's rather easy

On the picture you can see back and front of the two sleeping masks I've made so far...

Good luck with yours =)