Chain Link Joining Method
Your first square is the ONLY square that you will do this with. The rest will be joined as you crochet on one, two, or three sides.
Your first square will look like this when you are finished:
SECOND SQUARE: Repeat method for first square across two sides of your square. Stop after you have made the first sc in the third corner. Ch 2, join to corner of first square by making an sc in the corner ch-4 loop, ch 2, sc in same corner space of second square. Ch 2, sc into next ch-4 loop of first square, ch 2, sc into next ch-4 loop of second square.
Continue in this manner joining squares on the appropriate sides.
Note: When you get to the point where you have adjacent squares (they only touch on the corners), join to the adjacent square's corner ch-4 loop. This makes a more secure join and looks much neater.
I hope I have explained this adequately. I don't want to take the time right now to make a full-blown tutorial with pictures for each step. I did a brief look for this method on YouTube so I could provide you a link to it, but I didn't find it.
I hope this is helpful!