Pick up sts along the side of your work. Hold the yarn as if you would knit.
Put your needle through the holes inside your edge stitch.
Wrap yarn around the needle, and pull through.
You could also use a crochet hook to grab the yarn, and then put the st on the needle.
If you have worked your edge sts every row e.g in garter as on this piece, every hole represents 1 row.
If you intend to knit stockinette on your new sts, typically you pick up 3 sts for every 4 rows, garter 2 sts for every 3 rows and rib 1 st in each hole.