You can add a Single Crochet Border to anything, be it knitting or crochet. In my video I am working with a small garter stitch swatch that I knit.
I started a couple of stitches away from my corner, then began working a single crochet into each stitch. The corners require a little something extra to help the fabric lay flat. To work your way around the corners, all you need to do is work three single crochets into the same corner stitch. When you make it back to your starting point, just join the round with a slip stitch into your first single crochet of the round. Cut the yarn and weave in the tails. Or, you can continue working in rounds of single crochet for a wider border.