The main stitch of amigurumi crochet and the first and shortest of the basic crochet sitches.  It makes a firm fabric that does not have a lot of drap.  

Having worked a foundation chain or chainless foundation

Step 1.  Insert  hook into position you wish to make the double crochet.

Step 2. Yarn over hook and pull the yarn through.  You now have 2 loops on the hook.

Step 3. Wrap the yarn over your hook again.

Step 4. Pull the yarn through the 2 loops on your hook.

See turning chain for double crochet when working in rows.