http://www.blogcdn.com/www.joystiq.com/media/2006/10/mii.swf

1.  Create an avatar to look like yourself.

2.  Hold all three keys down at the same time.  "Shift" + "Apple Key" + "4"

3.  A plus symbol will appear on top of the mouse/hand.  Click and drag a box over your avatar.  Selet ONLY THE HEAD of your avatar.  NOT the whole body.

4.  When you release the mouse it will automatically save to your desktop.