Pick up Stitches
Adding a knit-on border to a finished piece of knitting can give a pattern made of multiple blocks more cohesion, or it can just make an afghan or blanket look more finished.
For the purposes of this instruction, we're putting a border on a small knit swatch, but the exact same rules apply when knitting a border on an afghan. (It's probably a method best reserved for a smaller lap and baby blankets, however, because a large number of stitches can get maddening really fast.)
To begin, choose the yarn that you want to use for the border, which can be the same as one used in the afghan or a coordinating yarn or color. Choose a circular knitting needle of the appropriate size for the yarn you're using, with a cable long enough to comfortably accommodate the number of stitches you'll have but short enough to work with without stretching the work too much.
Starting at any corner, pick up stitches along all four sides. On this swatch, we picked up one stitch for every other stitch or row all the way around, except for picking up more stitches around the corners.
Playing with borders on swatches is a great way to learn what you like in terms of how many or how few stitches to pick up. It's also a good way to experiment if you might want to do a border of a particular pattern stitch and aren't sure how it will look.
If knitting a basic border like garter stitch, the number of stitches you pick up doesn't matter, though there should be a uniform number on corresponding sides. If you're knitting ribbing or a pattern stitch, ensure you pick up the right number of stitches to make the pattern work.
Knitting the Border
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.
Finishing and Blocking
Once your border is worked to the length you like, bind off as you normally would. You'll notice when you take the work off the needles that it may not look very nice; it will probably be really rounded in the corners and maybe a bit puckery, especially if you bound off too tightly.
Blocking will help tremendously in making your corners a bit more square and giving the project the nice finished look you're going for.