Start the border in the extra corner stitch on the bottom left hand side with a standing single crochet, and single crochet in the next corner stitch.
*Chain 4, skip the next 3 stitches, single crochet in the next two stitches* repeat this pattern across the bottom edge until you reach the next corner (you should have 6 loops). Make a single crochet in all 3 corner stitches. Carry on the pattern above. Do this for 3 sides.
When you get to the top left corner, single crochet in the 3 corner stitches then chain 4. Pull up a loop, then insert your hook into the top right loop of the 1st square. Hook the loop from your chain 4 and pull it up through the loop of the 2nd square. Pull your loop tight, skip 3 and single crochet in the next 2 stitches of the 1st square.
You should have joined the first 2 loops together. Repeat this for all 6 loops until you get back to the bottom left hand corner. Single crochet in the corner stitch, and join into the standing stitch.
Once you have made a few squares joined together we can close the gap in the corners where 4 squares meet. This is done with a single stitch. You can see how its done in this video
You can now tie off and weave in your ends.