http://www.focusfanatics.com/forum/showthread.php?t=133486
This says 07, but AFAIK, the suspensions didn't change much in that department for the rear. Get under your car and see what you can see, compare it to this guide. Chances are the same principles apply.
So you will need a swaybar, (Get the SVT one 'cause it's the BOMB,)
Some endlinks,
Some bushings,
and someone who will weld the appropriate brackets to your rear subframe.
If you don't have access to someone who will weld, a muffler shop will probably do it for you.
You could go with Steeda's bracket-based adapter kit, but I would advise against it; the install doesn't allow the proper swaybar geometry, and once the brackets wear in and form to the contours of the subframe, it starts to clunk from the rear end all the time. Gets pretty annoying.