Double Crochet Thermal Stitch or Half Double Crochet Thermal Stitch are exactly the same as (single crochet) Thermal Stitch, except worked with dc or hdc respectively.

I have a fully detailed written and pictorial guide on how to do the default (single crochet) Thermal Stitch.
originally called it the Strap Stitch because I “invented” it while trying to make a sturdy strap. I suspected but didn’t know then that such a stitch already existed. Now it is just too much work to try and change all my earlier references.
Here, I am sharing a video tutorial (by someone else) towards the end of the page.
dc vs hdc Thermal Stitch
In the video, he only talks about the dc thermal stitch. But the hdc can also be used in the Thermal Stitch method. In fact, I prefer the hdc Thermal St.
Tip: How to Get Straight Edges

As I detailed in the Strap Stitch/Thermal Stitch (sc) tutorial, the trick to getting straight edges is:
Skip first stitch, add one stitch at the end.
How to Crochet the DC Thermal Stitch
Note:
i would recommend you read through the basic (sc) Thermal Stitch tutorial first. That should give you a much clearer idea of what is going on when you watch the video below on how to crochet the dc Thermal Stitch.
i would recommend you read through the basic (sc) Thermal Stitch tutorial first. That should give you a much clearer idea of what is going on when you watch the video below on how to crochet the dc Thermal Stitch.
(by New Stitch a Day: Knitting and Crochet Video Tutorials)