Now that you have all the stitches you need for your border, it's a simple matter of actually knitting the border in whatever pattern stitch you like.
Remember that you are now working in the round, so to get a Garter Stitch border as shown you actually have to knit one round and purl one round.
Use the tail end of your yarn or a stitch marker to note the end of the round.
The number of rows you feel your border needs will depend on the size of the afghan and your preference. Mine is four rows long, not including the pick up stitches row or the bind off.
