Once you remove the rear bumper cover (required for 2011 to 2012 models), the three center tabs of the lower valance, right below the license plate can be a pain to pull off. Be patient and push them out with your thumbs. Many cut these three off, but then you'll not be able to re-sell it, or re-install the original if you sell the vehicle and wish to return it to stock (to sell off the mods).
I just installed a GT500 rear valance on my 2011 a few weeks ago. Did it myself, but a second set of hands would have made the install a bit easier ("gravity" makes the removal easy, installing required a bit of careful alignment with only two hands).
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.