Double crochet stitches in standard crochet (i.e. “non Tunisian”) are worked as follows:
yarn over, insert hook into the work, yarn over and pull up a loop, then (yarn over and pull yarn through 2 loops) twice. There is only 1 loop left on the hook before you start the next double crochet stitch.

Tunisian double crochet stitches are worked exactly the same way:
yarn over, insert your hook into the work, yarn over and pull up a loop, then yarn over and pull yarn through 2 loops. This last movement is done only once. Each stitch worked on the FwdP keeps its final loop “active” (“open”) until the end of the FwdP. All these loops accumulated on the hook are closed on the RetP.

The way you insert your hook into the work can be done in any way you would do for any basic stitch. In other words, you have at least 4 variations of the Tunisian double crochet stitches: simple, knit, full and reverse.

Tunisian double crochet simple stitch

Yarn over, make a simple stitch, then yarn over and pull yarn through 2 loops.

Tunisian double crochet knit stitche

Yarn over, make a knit stitch, then yarn over and pull yarn through 2 loops.

Tunisian double crochet full stitch

Yarn over, make a full stitch, then yarn over and pull yarn through 2 loops.

Tunisian double crochet reverse stitch

Yarn over, make a reverse stitch, then yarn over and pull yarn through 2 loops.

Note on edge stitches

Just as for extended Tunisian stitches, Tunisian double crochet stitches are higher than their equivalent basic stitches. You need to adjust the height of the edge stitches at the start and end of rows.

At the start of the FwdP, before picking up your first Tunisian double crochet stitch, when you have only one loop on your hook, make one chain.

At the end of the FwdP, when making the last Tunisian double crochet stitch into the edge stitch of the previous row, insert your hook under the 2 strands on the side, like you would for any edge stitch at the end of the FwdP.

At the start of the RetP, start with one chain, then make a yarn over and pull yarn through 2 loops.