As we are joining as you go, we need to add the interlocking border this border weaves the loops of 2 squares together to create a woven look that is very forgiving if your squares come out slightly different sizes. Use a 4mm hook for this part of the border on all the squares in the crochet along.
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.