Sew the small side panel to the inner facing piece as shown, aligning the small dots.
Sew the panels together leaving a gap unstitched between the two dots and press seam allowance open.
Edge stitch the filter opening to hold seam allowance in place.
Next, align the inner facing pieces and outer mask pieces with RIGHT sides together and sew along the curve with a ¼” seam allowance.
Sew the ends of the elastics to the sides (on the RIGHT side of the fabric) forming loops as shown.
Align the two layers (inner facing with filter opening and outer mask pieces) with RIGHT sides together and stitch all around the perimeter with a ¼” seam allowance.
Clip the corners to reduce the bulk and turn the mask inside out through the filter opening.
Press the seams flat with an iron and edgestitch around the perimeter.