Starting with my favourite, if a little more time consuming finish, the zipper stitch. You simply lay your squares facing right-side up, and do a slip stitch through the aligned back loops.
STEP 1 Lay your squares next to each other right-side up, lining up the corners and edge stitches.
STEP 2 With your slip knot on your hook, holding your yarn to the back of your work, insert your hook into the back loop of each square, one at a time.
STEP 3 Yarn over and pull through both back loops, and the loop on your hook.
STEP 4 Repeat the process to the corners - I like to alternate each square I go into first - insert your hook into the corner-spaces, rather than into the chain stitches.