You will need:
Medium weight fusible interfacing (for stiffness)
Cut two 11x11cm fabric squares. Put the right sides of the fabric together and stitch around the edge leaving a 1cm margin. Leave a gap of about 1 inch along one side.
Cut the interfacing to 9x9cm and iron onto one side of the mat. Using the gap, turn the coaster right side out, and iron flat making sure that the open inch is tucked in. Topstitch around the coaster a few millimeters in from the edge to finish.
I'm planning to make some Christmas ones next!