Assuming that you already have a working row (not a base row):
Step 1: Loop yarn over hook
Step 2: Insert hook (from front) on the side of the stitch you are currently working on. Move hook around the back of the stitch and out the other side.
Step 3: Yarn over hook and pull loop through.
Step 4: Proceed as how you work on a dc (yarn over, pull two times).
If you wish to make it a front post treble crochet (fptr), instead of working on a dc, just work it as a tr. Same concept applies for back post double crochet (bpdc).
Here's a look of the stitch along side a regular double crochet.
|Top: back view of an fpdc. Bottom: front view of an fpdc.|
An instruction on how to make an fptr, fpdc, and bpdc are included in the pattern linked at the beginning of this post. When I first read it, I got confused and did it wrong. That is why I want to provide this visual step-by-step guide so you won't get confused like I did.