You will be decreasing 2 stitches, one on each side of the ankle. These decreases occur every three rows. So there will be one decrease round and two plain rounds.
Decrease round for magic loop: Starting at the center marker, knit to the last 3 stitches on the needle, k2tog, k1. Knit 30 instep stitches onto second needle. Go back to first needle. K1, SSK, knit to the marker -- 2 stitches decreased.
Decrease round for socks on DP needles: Needle 1: knit to last 3 stitches on needle, k2tog, k1; Needle 2: k30; Needle 3: k1, SSK, knit to end of needle.
Rounds 2 & 3: Knit -- no decreases.
Repeat these three rounds -- decrease, knit, knit -- until there are 60 stitches.