Double crochet is a taller stitch than single crochet. It is formed by a "yarn over," which is wrapping yarn from back to front before placing the hook in the stitch. Holding foundation chain in desired position, yarn over and inset hook into the 4th chain from the hook.
Yarn over and pull through chain. You should have 3 loops on hook.
Yarn over and pull through 2 loops on the hook.
Yarn over again and pull through remaining 2 loops. One double crochet has been made.
Work one double crochet in each chain to the end,
At the end of double crochet rows, chain 3 stitches to achieve the same height stitch as a double crochet. This is called a turning chain and it counts as the first double crochet on the next row, so the first double crochet is actually worked in the second stitch.
© Reprinted with permission from Craft Yarn Council of America's Certified Instructors Program