for the ones DH takes for his lunches, and for daily use in our household, I made a bunch of them from old bath towels that had worn thin or were too shabby for other use. I also cut some from a large old terry bathrobe. Just single layers.
For the "company" ones, I cut flannel squares from my stash and used two layers, having the right sides facing out.
All of them were serged around the edges with a variegated thread.