You can pick up stitches across the long side and knit until you have the right border depth. Do the other long side. Then pick up stitches across the top and the bottom and do those. If you're still learning crochet, I'm sure you're skilled enough to do a simple single crochet border all along it. When you do your border, whether it's knit or crochet, be sure to use the same size needle or hook as you used for the body of the blanket.