Back post stitches appear to recede on the side of the crocheted fabric that's facing you. You can create a back-post double-crochet (abbreviated BP dc) stitch with these steps and some practice.
Work a row of normal double crochet (dc) for the first row and turn.
Follow the standard procedure for creating a row.
Chain (ch) 2 for your first double crochet.
Because a post stitch is shorter than a normal stitch, you make the turning chain with one chain stitch less than the normal turning chain requires.
Yarn over (yo) and insert your hook from back to front between the posts of the first and second double crochet in the row below, and then from front to back again between the posts of the second and third stitches.
The hook should now be positioned horizontally in front of the double crochet that you’re working around.
Yarn over and draw the yarn around the post of the stitch.
You now have 3 loops on the hook.
Yarn over and draw the yarn through the 2 loops on the hook, twice.
You’ve made one complete back post double crochet (BP dc).
Take a look at the symbol for back post double crochet.
Stitch diagrams use symbols to give you a pictorial description of the pattern design — and may or may not include written directions. This is the symbol for back post double crochet in crochet stitch diagrams.