Start by cutting two pieces of fabric. You can cut one on the fold, but it's better to use contrasting fabrics so the wearer knows which side goes out.
Next hem the top edge of the outer fabric. Leave a gap in the center for a removable nose wire.
Serge the top edge of the lining.
Serge together the two mask layers at the bottom, wrong sides facing. Top edge is left as is to form a pocket.
For cloth ties, use 1" wide strips (cut selvage to selvage). Begin serging down the strip, then when you get to the center of the strap placed the pleated mask under the strap. Keep sewing until you reach the end of the strap. The sides should be under 3" once pleated.
If using twill tape or bias strapping, simply serge your sides with 3 pleats then straight stitch the straps down (two pieces, each 36" or longer)
When using ties you need to tie off the ends or they will fray
For a tighter fitting mask, add a dart to the chin.
Too advanced? DIY Home has a good tutorial with easier versions.