Line up the tabs and pop it back in. Taking it out can be a pain because you have to tug in a quick forceful motion and the grille can be hard to grip. One word of advice...once u get it off do not try to separate it from the chrome surround. Just tape it off and spray.
You can also plastidip the grille without taking it off. There's a video of it, I think on dipyourcar. Similar technique to doing the wheels by placing a plastic bag behind the grille to protect those parts. Also, tape newspaper to the bumper and hood.