How to crochet a front post stitch
The first picture here is how things will look when you insert your hook to work a Front Post Double Crochet, usually abbreviated “Fpdc”.
You insert your hook from the front of the fabric on the right side of the post of the designated stitch, and then go across the back of the post, and go back through the fabric on the other side of the post. Then yarn over, and pull up a loop, then yarn over and pull through 2 loops, yarn over and pull through 2 remaining loops. So the only difference between a post-stitch, and a regular stitch, is that you work it around the post instead of into the tops of the stitches.
This photo shows how things should look when you are inserting your hook for a Back Post Double Crochet, usually abbreviated “Bpdc”. You insert your hook starting on the back of your piece. Go through the fabric and around the post and back through the fabric, so that most of your hook is behind your swatch.
Using crochet post stitches in cables
Typically, when working cables or ribbing, the non-post stitches are shorter than the post-stitches. Often, when working intricate cables, the non-post stitches are half-double crochet, and the post-stitches are double and treble crochet, depending on how slanted they are. The reason for the different sizes is that the bottom of the post-stitch is lower than the bottom of all the non-post stitches in the row. In order to make the tops of all the stitches in the row more-or-less even, the post-stitches need to be longer to make up the distance.
If you’ve never tried post-stitches before, now you are ready to give it a shot! Go for it! I hope that this answered your questions about post-stitches. If not, leave me a comment with your question and I’ll do my best to answer it for you.