This is very easy. I took a white tile that my grandparents used to have. I think of them every time I see the pot coaster.
Pick a tile that you like and make sure it's clean on both sides.
Open a pack of Sugru in any color and knead it until it's soft.
Split it in three or four parts depending on how many feet you want the coaster to have. Three is easy to make level but four, one in each corner, gives better support.
Roll the Sugru bits so they are round and press them to the bottom of the tile so they get flat and has a good grip on the tile. Let it set for 24 hours.
That's it. Sugru is pretty heat and water resistant so you can run the pot coaster in the dishwasher when it needs cleaning.