The idea is basically what's shown on this video
for booties. The knitter in the video picks up from the side of the instep whereas your directions are to start with a new end of yarn at the corner where the heel meets the rest of the sock but the idea is basically the same. Pick up the 15sts along the side of the heel. knit across the heel sts and then again pick up 15sts on the other side of the heel.