The single crochet join is pretty straightforward. If you already know how to do single crochet stitch, you know most of what you need to know to do this join; the only trick is getting it started. I demonstrate that for you in the free tutorial. I hope you'll find it helpful.
Also, if you like the squares shown in the photo above, I invite you to grab the free pattern for them: check out the scrap afghan square pattern.