Here are some more detailed instructions on how to make the cut-and-sew sleep masks from my Spoonflower shop (can also be bought pre-cut with wadding and elastic included, in my Etsy shop)
Assemble your materials. In addition to the fat quarter sleep mask fabric, you will need:
fusible batting, aka wadding, to make the mask nicely padded and help block out light (alternatively use another material such as felt)
pencil or fabric chalk
fabric scissors
elastic to fit around the back of your head from temple to temple (I've used a lace elastic here for extra glam, but you can use any elastic)
needle and thread
pins or sewing clips
(and ideally a sewing machine to make this the quickest and easiest way)
Cut out the front and back pieces. Use the pieces as a template to draw around onto the fusible batting - cut out the batting to match the front and back pieces
Fix the batting to the back of the pieces - I recommend using a damp cloth between the iron and the material. If using velvet or minky, press from top to bottom to avoid the nap of the fabric from setting out of place
Place your elastic horizontally on the front piece and fix each end to the dots on the left and right sides (if using a wide piece of elastic, use the dots as a guide for where the middle of your elastic should be) Keep the elastic laying horizontally across the material, folding it over itself in the middle to keep it inside the panel:
Place the mask pieces right sides together
Pin or clip together, ensuring the elastic is well within the 'sandwich' and laying straight.
Leaving a gap at the top of the mask for turning, sew around the edges using a quarter inch seam allowance
Remove the pins and snip notches into the seam of the mask
Turn right sides out, through the gap left at the top of the mask. Use a pencil or point turner to push out the curves for a neat finish
Fold in the top edge of the mask and sew closed using a ladder stitch
Done!