sew 2 placemats together on 3 sides (2 short, 1 long.) box the bottom the width of the doubled placemat border/ prior to sewing the mats together, use napkins to make inside pockets. attach straps--i would put batting in the straps for comfort. use a firm plastic or cardboard piece for bottom of bag (can be covered in napkin fabric which is removable.)
that's my best guess. doesn't look difficult at all.