A single crochet border adds a finished look to a piece of knitting, and is pretty easy to do even if you don't have a lot of experience with crochet. For the purposes of this tutorial, a contrasting color border is being added to a swatch of Stockinette Stitch. If your project has a lot of curl a single round of crochet might not solve the problem, but it still gives a nice look to the edge.
To begin, you'll need a crochet hook and some yarn that's the same weight as your project. Crochet hooks are sized with both letter and number sizes, but they are sized by the millimeter just like knitting needles, so you can easily choose a crochet hook that's as close as possible in measurement to the size knitting needles you used.
Start with the front of the work facing you. You can start your border anywhere you like, but I prefer to start toward the middle of a side rather than in the corner. Wherever you start, put the crochet hook into the first stitch from front to back and put the yarn over the hook (in crochet, this is sometimes called "yarn over").