For this example I'm using a size G 6/ 4.00mm hook and 100% mercerized cotton yarn(pic 1).
ch = chain
sl st = slip stitch
hdc= half double crochet
Ch 20 Join with a sl st to form ring(pic2).
Round 1 Ch 1 , work 20 hdc in ring (pic 3), sl st to join to the ch 1 worked at the beg of the round(pic 4).
Round 2 Yarn over, insert hook from back to front around next hdc. Yarn over, pull it through all 3 loops (pic 5). Yarn over, insert hook from front to back around next hdc. Yarn over, pull it through all 3 loops (pic 6). Repeat until round 2 is complete. Now this is how it would look after you're done (pic 7).
Want to know how it would look when repeating only back post hdc? Check out pic 8 and 9. For front post hdc, picture 10 and 11.