Write an article based on this "Work a preparation row using the Tunisian simple stitch. Chain two. Forward pass into the second vertical bar. Work across the rest of the row. Reverse pass through one stitch. Reverse pass through the rest of the row as usual. Repeat as needed."
The Tunisian double crochet starts after you have completed a preparation row using the Tunisian simple stitch.  You can also work the Tunisian double crochet into larger piece of Tunisian simple stitches. The preparation row is a minimum start, but not a maximum start. Make sure that you have completed a reverse pass before you start the Tunisian double crochet. There should only be one loop on your hook when you start. Work two standard chain stitches from the loop on your hook. These chain stitches will help accommodate the eventual height of your Tunisian double crochet row. Yarn over the hook once, then insert it into the second vertical bar. Yarn over again, then draw this yarn-over back through to the front of your work, creating a loop. Yarn over once more, then draw your last yarn-over through two loops on your hook.  Note that the first vertical bar should be skipped over, as done with the simple stitch. Leave the final loop of the stitch on your hook. There should already be one loop on your hook from before that, however, giving you a total of two loops on the hook at the end of this first double crochet. The difference between the Tunisian simple stitch and the Tunisian double crochet lies entirely in this forward pass part of the process. Repeat the previous step, working into each vertical stitch of the previous until you reach the end of that previous row.  For each stitch, yarn over the hook once, insert it into the next vertical bar, and yarn over again. Draw the yarn-over back through to the front, yarn over again, and draw this last yarn-over through two loops on your hook. For the last vertical bar, insert the hook into the horizontal stitch lying to the right of the vertical bar as well as the vertical bar itself. When pulling a loop back through to the front of the work, make sure that you pull it through both bars again. This adds stability to the edge of the work. When you reach the end of your forward pass row, you should have 10 loops on your hook, or the number of stitches you started with in your foundation chain. Yarn over the tip of the hook and draw that yarn-over through one loop previously on your hook. Note that the reverse pass for the Tunisian double crochet is the exact same as the reverse pass for the Tunisian simple stitch. Yarn over the hook, then draw that yarn over through two loops on the hook.  You should be left with one less loop on your hook at the end of this step. Repeat this step until only one loop remains on the hook. Alternate back and forth between the forward pass and reverse pass until you reach the end of your Tunisian double crochet section or the end of your work as a whole.  Always end with the conclusion of a reverse pass. Skip down to the section on "Finishing the Work" if you are ready to tie off the work at the completion of this step.