Start out with a square of fabric. You can really make these big or small. I made 16" squares.
Once you have squares you can start pinning the hems on the sides. Be sure to fold it over twice, so it doesn't fray too much. (Look at pics below.) Then just pin around the whole square.
Finally, sew around the whole square and viola! You've made a napkin that's green, reusable and cute! :)