The double crochet stitch is also a very common stitch; it is twice the height of the single crochet.
1. Place yarn over hook first, and then insert hook into the back ridge of the 4th chain stitch from hook (not counting loop on hook). Do not draw yarn through yet.
2. Place yarn over hook again and draw yarn through the chain stitch that hook was inserted through. There are now 3 loops on hook.
3. Place yarn over hook again and draw yarn through the first 2 of the 3 loops on hook. There are now 2 loops on hook.
4. Place yarn over hook again and draw yarn through the 2 loops on hook, completing one double crochet.
5. To continue, insert hook through next chain and repeat the steps above. To make a row, insert hook into the next chain after completing each double crochet and repeat the process.
6. At end of row, make a turning stitch (step 5 in single crochet instructions, except ch 3 instead of 1); then, working in the opposite direction, work a double crochet in each stitch of previous row. The turning ch 3 at end of previous row counts as the first double crochet of next row, so begin the double crochets of new row in the 2nd stitch of previous row. When working subsequent rows, work the last double crochet of the row into the 3rd chain of the “turning ch 3” of previous row, in order to keep work even. Then ch 3, turn work, etc.
Source: Christmas Time Crochet, Craft Publications Volume 7317
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