It's not hard to find materials around your house that will work perfectly for your mask, even if you don't have a giant closet full of fabric. I used a holiday dish towel someone gave me for Christmas for the liner layer, and a t-shirt with unicorns on it for the outer layer. 

The important thing is to use fabric that breathes -- cotton or other natural fibers are your friend here. Hold it up to your face and try breathing through it before making your final decision.

And, use tightly-woven cotton fabric on both sides - so no gauzes.

Choose which size you want and cut both patterns out along the correct line. 

Lay one piece face up and one piece face down on your lining fabric. Leave a little space between the pieces. Trace around them with a marker, then draw another line 1/4" away from your first line. This is our seam allowance -- we'll trim it down a bit later, but it's there to give us a little more room to work.

Cut both pieces out along the outer line.

Now repeat with your outer layer fabric. Take a minute to line up the pieces so the design lays out nicely on the front of your mask. Remember to have one piece flipped upside-down so you have a left and right side. And remember that the curved side will be the middle of the mask, so if you want the design to span both sides, keep the curved sides inwards.

Once you've got all four pieces cut out along the outer cut lines, it's time to sew it all together. Thread your needle with about two feet of doubled-over and knotted thread.

Place the inner liner pieces on top of each other and stitch along the long curved edge (the part in the center in the above photo). 

For the outer front piece, stack the two pieces with the picture on the inside (right sides together) and stitch the same front edge.

Trim the front seam allowance of both pieces close to the seam you just made, being sure not to cut through the stitches.

Open both pieces up. Stack them on top of each other with right sides together (so your seams are both on the outside). Sew along the top edges and the bottom edge, leaving both side edges open. 

Trim the seam allowances you just made close to the stitching.

Turn the mask right-side out so all your seams are now on the inside. Fold both edges to the inside. Iron it flat. I found it worked better to iron on the towel side rather than the t-shirt side, since I had printed designs on my t-shirt that threatened to melt if I got them too hot.

Slip an elastic hair tie in between the two layers on each side, so it's just barely sticking out. Sew the edge closed around the hair tie, being careful not to actually stick your needle through the elastic -- just sew around it, to minimize pulling the threads when you stretch the mask on.

Try your mask on! If it's too small, you can tie another hair elastic on to each side to make it larger.