At corner C you do the same thing as with corner B. You will also make a temporary loop here, you will do two single crochets in it, of which the first is the last stitch of the second side with crochet stitches, and the second is the first stitch of the third border. Mark these stitches, or at least the first one of the side, because it facilitates counting. The pictures:
Again, make stitches in each sideloop till you have come just before corner D.
Corner D is very simple. You already have a loop around the corner nail there, so you just work through that right away. Make 2 stitches in this corner loop!
And again make stitches in each border loop until you have come just before corner A.
Go to the next page for explanation how to end at corner A. The way to work there is important!