Select wich fabric you would loe use, maybe some pattern to bother or contrasting pattern.
Cut 2 (4 x 4 cm) squares pieces of fabric and 1 (4 x4 cm) piece of fusible fleece.
Layer the squares as follow fuible fleece (back & front).
Ensure the patterned pieces the right sides each other, pin the pieces together, sew 1/2 cm seam around the edges of the squares, leave opening sides as you’ll need, turning the coaster the right way.
Trim the corners and sides approximately 1/4 cm to the stitched to ensure I not bulky
Leave the side opening as your need enough sew closed it, turn the coaster to the right side through the opening and press it with the iron, sew the opening closed with small stitch.
When sewing around the edges, make sure the needle is down through the fabric.