Ch = Chain
Sl St = Slip Stitch
Sc = Single Crochet
Dc = Double Crochet

1. Ch 5, join in beg ch to form ring, ch 3 (considered as 1 dc) , 11 dc in ring, join in beg dc. (12 sts)
2. Ch 1, 2 sc in each st around, join in beg sc. (24 sts)
3. Ch 1, 1 sc in same st, ch 2, skip next st, [sc in next st, ch 2, skip next st] around, join in beg sc. (36 sts)
4. Ch 1, [3 sc in next ch-2 space] around, join in beg sc.  Fasten off. (36 sts)

Optional (if you want coaster to be a little bigger):

1. Ch 2(considered as 1 hdc), hdc in each st around, join in beg hdc. Fasten off. (36 sts)