Stack both the pieces of fabric on top of each other. Fold over the long side by 1/4" and hem. Similarly, fold over the shorter side by 1/2" and hem.
You should now have a double-layered stitched rectangular piece that is open at the top and the bottom on the shorter side. Use a large needle or bobby pin to insert the elastic through the open ends at the shorter side. If you don't have elastic, use a rubber band or hair tie.
Knot at the ends of the elastic to make a loop, which will fit around your ears. Alternatively, you can use a string. In that case, tie the string behind your head. Make sure that the elastic loops are snug and the mask fits your face closely.