If you want to install a black mesh, like a grillecraft, then you're going to have to cut out the center with a hacksaw or something like that to get it in. There are some grills that you can buy that are alread complete, and you just pop the stock one out and put the new one in, and even ones that you can snap over the stock. It all depends on you and how you want it to look. I cut out my stock center and inserted a grillcraft mesh.
This is the aftermarket one that I bought off of ebay.
I didn't like it so I cut out the center part of the stock grill, added a decepticon logo and this is what it did
and this is how it looks now
Hope this helps.