There are lots of great mask tutorials out there right now. Here's how I'm making mine!
STEP ONE: gather your materials--two 8"X8" pieces of quilting cotton fabric, one piece of 8"X8" light to mid-weight fusible interfacing (if you don't have this, don't worry--it's not essential, just nice for adding extra protection), two 13"-long pieces of 1/4"-wide elastic (if you don't have elastic, cut four 18"-long, 1"-wide strips of an old stretchy t-shirt--see picture below)
NOTE: This is sized for an adult's head. To make one for a child, adjust your materials like this: 7"X7" fabric squares, 11" elastic pieces or 16" ties.
STEP TWO: If using interfacing, fuse the interfacing to the WRONG side of one of your quilting pieces. Have a parent help you with this step! If using ties instead of elastic, stretch out the strips so they roll up nicely into little tubes.
STEP THREE: Angle the end of one piece of elastic into one corner of your cotton fabric (right side up!). Then place the other piece of cotton right side down on top of the elastic, as shown in the picture. Pin securely in place.