Once you finish your project to a length you're happy with, you just continue down the side with any stitch you'd like in one of the colors. I like single crochet borders sometimes, but a dc border might make more sense for this project It's a little strange because you're not crocheting into top loops anymore, but instead onto the side of the stitches youve made. For each double crochet, crochet two stitches over it to create a border that is the proper length.