It seems that the pattern starts with knitting the left heel. When all the sts were on one needle, you finished with a knit row so that the next sts would be the purl row. The directions for the left heel start out with that purl row using the strand of working yarn that is coming off the needle tip. You'll purl 13sts (or in your case 15sts) and turn. So that will define the left heel. It'll be the 15sts next to the working strand of yarn.
Thanks for the pattern link. Those stockings look like fun to knit.