Thought it might be easier to have it all here, then it can be deleted easily 
1st. Cut the 3 layers of fabric 9" x 7" plus 2 pieces of elastic (about 1/8") at 6 1/2"

Layer the fabric interfacing on the bottom, then outside  side of mask, right side up, and the back right side to the mask. CAREFULLY pin the elastic to each short side about 1/2" from the end -

The fabric will bunch up.
Pin all 4 sides,  and mark about 2 1/2" on one long side - it will look like thisi 

Sew from the "S" right around, making sure you have caught the elastic in, and reverse over the elastic
1/4" seam all around till you get to the end "F" mark

Clip the ocerners off, and turn it right side our.

Press nicely, tucking in the raw edges.

THEN press in half,  wrong side together 


NOW bring each of the bottoms up to the 1/2 way,  this time the RIGHT SIDES TOGETHER, .it will look like a concertina fold, like paper fan making

Now you're almost done, pinch each  one of the lower folds, and bring it to the centre fold, and pi onto place.
I pinned on the back for the pic,, usually pin at the front, but hard to see with the dark fabric, 

Now sew all around, about 1/8" from the edge, I usually start just below the first fold, and reverse over the edge of each  fold, this will also close the gap left on the long side.


You can easily see where I didn't exactly sew on the fold 

And you're finished,


Hopefully I've explained it well enough, ask away if you need any further pics or comments