Back Post Double Crochet:
1. Yarn Over Hook.
2. Tip your work over, as if to peek at the other side, insert hook from back to front, right beside the stitch you're going to use (according to the instructions in your pattern).
3. Go AROUND the stitch until you come to the back side of your
work again, then Yarn Over Hook. Pull through around the stitch again
and pull it up, towards you. There should be three loops on your
hook. Finish your back post double crochet (bpdc) as you would a
normal double crochet.
4. This is what your work should look like from the front:
