Make a slip knot and chain 4
Join the chain with a slip stitch to form a ring
Chain 2
Work 12 double crochets into the ring
Join with a slip stitch to the top of the starting chain 2
Chain 2
Work 2 double crochets in each stitch around
Join with a slip stitch
Chain 2
Work 1 double crochet in the first stitch, 2 double crochets in the next stitch
Repeat around
Join with a slip stitch
Continue increasing evenly in each round until the coaster reaches the desired size
Fasten off the yarn
Weave in the loose ends