Based entirely on this pattern from Gather Here, but simplified somewhat (and without the option to insert a filter).

Start with a 7″x19″ piece of quilting cotton:

Press 2” of fabric in by folding the right side of the fabric together. After pressing, bring that fold to meet the center 3.5” line and press to create another fold. Repeat on the other side, you should have a box pleat on the right side of the fabric.

Here’s where my instructions diverge from GH’s.

Mark the center line of the mask, and sew the pleat down.

Fold right-sides-together:

Sew the edges with a half inch seam allowance, leaving an inch unsewn at either end:

Invert the bag, and re-form the pleats:

Fold the raw edges inwards:

Pin and sew close to the edge:

Measure out ~28-30″ of elastic. Use a hook to pull elastic through the small gap left behind:

and then through the other side:

Tie the elastic in a knot, and then it can be worn in the “above and below the ears around the back of the head” style. If you prefer the ears style, use two shorter pieces through the ends of the mask.

If you have glasses, you can use a short pieces of bias tape or cloth tube and sew it to the bridge of the nose:

and then thread two pipecleaners through it which allows you to mold the top well enough to help prevent glasses fogging.