When you knit in garter stitch, it creates 'ridges'--the lines of knit rows and purl rows. (Even though you knit every row, the back of a knit is a purl and the back of a purl is a knit, so when the fabric faces you, you see a row of knit and a row of purl.) The purl rows are the higher lines and the knit rows are the dips between them.
On the edge of your fabric, you'll have 3 lines of purl and 3 lines of knit. Pick up a stitch in each of the 'dips' of knit rows.