Choose two fabrics in colors you love, one for the star and the other for the outside frame and backing. The backing is turned to the front to create the binding so a third accent fabric could be used for the backing if you prefer.
The star appears when folded triangles of fabric are affixed to each side of the hexagon. Cotton batting and Insul-Bright provide heat proofing.
In the instructions, from Raewyn of Stitching Farm Girl, an 8 1/2″ hexagon is used as the base (which is approximately the finished size). However, you can make your trivet whatever size you want. Small ones will be lovely coasters, while large ones can be place mats.
Use a hexagon template you already have or create one in various sizes with the PDF found here from Kati of From The Blue Chair. Kati provides measurements and instructions for cutting hexies in many other sizes here.