You're going to need 2-3 skeins of contrasting colored yarn.
Start by putting your hook through the corner hole where you initially started or ended the blanket. You should have a 3" or 4" tail from starting the blanket. Just lay the tail to the side, we're going to stitch over the top of it and it will lay inside the border.
Then, start a slip knot loop with your border color and put it on your hook.
Pull (or slip stitch) the knot inside the hole...
Now you're going to dc in the holes which are actually the ends of the rows in the blanket. You're going to stitch about 6 dc's in every row of your blanket. You can see in the picture here that I'm stitching inside the edges of where my blanket rows began.