This is the symbol instructions for the Ripple Crochet. The multiple is 12 + 6.
The center valley sections are a double crochet decrease. It is done by doing half of your double crochet, leaving two loops on your hook. Skip the previous decrease and continue with the other half of your decrease. You should have three loops on your hook. Pull the thread through all three loops on your hook to finish the decrease.
Ripple Crochet Video Tutorial:
Ripple Crochet - Variation 1
Here is a video demonstration for decreasing double crochet if there is any confusion.