Wrap your yarn around the hook once before you insert the hook into the previous row of stitches.
Place your hook down through the hole, wrap your yarn and bring it back up through the same hole. You will now have three loops on your hook.
Wrap yarn around your hook again above the three loops.
Pull through two of the three loops on your hook, but not the last one. You will now have two loops on your hook.
Wrap yarn one more time.
Pull through the remaining two loops on your hook.
There you have it, a double crochet!