I removed my rear bumper/valance on my '11 to remove its lower valance and install a GT500 one. It was easy to do, except for the 3 tabs right above the license plate; they required a bit of tug and push.
I simply Googled "rear valance removal" and found a few how to links from Roush and others that sell rear valances.
Here's a link that has a free PDF file that provides a step by step on how to remove then install:
How to Install a Roush Rear Valance for a 2010-2012 Mustang
Some will say that the removal of the rear taillight housings are not required, but I did it (took an extra 10 minutes) which allowed the bumper/valance install easier to do without worry of scratching the paint on the upper bumper by the light housings. Give yourself about two hours for the complete removal/install. An extra person to help with the install helps too, but not required.