Here is a photo of a typical mask: http://www.lakewoodconferences.com/catal...isposables.html

You'll notice that there are two pleats on each side. They are about 6.75" square (with hems -- they probably start out about 7.75" square), and end up being about 6.75" x 3.5" with the pleats.

A tightly-woven plain cotton muslin would probably do, but remember to pre-shrink the fabric in a hot soapy wash before you cut the pieces to size. Muslin usually comes in two basic colors, beige and white; if you made each mask of at least two layers, making the inside (to the face) white would clearly designate which is which.

The ties could be just lengths of bias tape, stitched along the open edge, made long enough so you can tie a bow.

Sue