This is a fun border to add to knit or crochet items. It can be awkward at first, but with practice it will become easy.
Reverse Single Crochet or Crab Stitch is worked from left to right, instead of the normal right to left direction - if you are right handed. If you are a leftie, you can find some info here that may be helpful.
Make sure the front side of the item you are working with is facing you, and starting a few stitches from the left most corner, insert your crochet hook and pull the yarn through. Chain one. (This is my personal preference for starting the Crab Stitch. You may begin right in the corner if you wish, and you do not have to chain one. I just find it helpful to do so. I also find it helps get me started if I work a round of regular single crochet. You can begin working directly on your item.)
If you are planning to work this border with the same yarn you are using for the main body of your project, all you need to do is start working backward after the last row instead of turning your work as normal.
You'll want to insert your crochet hook from front to back. The hook should be pointing to the right. Grab the yarn and being to pull through. As you are pulling the hook through, swing or twist the hook so that it comes out pointing to the left. Be careful not to pull the yarn through the loop that is already on the hook. Now, yarn over and pull through both loops.
Here are some illustrations of this stitch, with the symbol.