Slip Stitch & Double Crochet
You can also join your squares using crochet stitches on the back side of your work - or in the case of the double crochet on the front too. Working the double crochet stitch forms a ridge on the work, which will often lean in one direction, rather than sitting flat up.
STEP 1 Hold your squares right-side together, lining up the corners and stitches.
STEP 2 Start with a slip knot on your hook, then insert your hook into the tops of the stitches, and work along, stitching into the top of each stitch and into each corner-space.
STEP 3 For double crochet, you may also want to start with wrong sides together.
STEP 4 When you reach the corner joins, decide how you want the crossovers to sit. Working logically and consistently over or under.