wow, I actually searched and FOUND something!!! Miracles do happen!!! LOL.. Here is a link to a tutorial for this type coaster. The only difference is they don't do the seam around the outside to finish it off. I prefer that nice touch, myself. You can add it or not as you please.
http://allsorts.typepad.com/allsorts...ross_coas.html
PS: There is some discussion as to whether to add something between the bottom and lining to absorb condensation. I did use a 3-3.5" (varied because it was scraps) piece of left over batting in mine.