These are the steps that I took to paint my grille:
Thoroughly clean the grille(make shure there is no oil or grease on it)
Sand it. this step is going to decide how well your finished result will be. Put lots of time and effort in to the sanding and prepping and your grille will be a beauty. For your paint to stick well it is usually recommended a 400 or 600 grit (bigger number finer the sand paper)sand paper. It is good to always do wet sanding because it prolongs the life of your sand paper and dosent allow dust particles to get in the air. If it is really bad, then start with a very coarse sand paper, you can also try using power tools(you have to get creative.)
you have to clean the grille again, because you can't leave any dust or dirt between the paint and the grille.
Now you can start with the cool part. You have a choice now. you can use a primer under the paint, or you can just paint it. I recommend using primer, because it is a very good medium for the paint to properly stick.
when you are going to paint you have to paint in a good sunny day with low humidity.
Paint the grille with primer, all of the coats have to be LIGHT if they are too heavy you can end up dripping paint, let 5 to 10 minutes between coats. after you finished applying the last coat of primer, wait 15 min then start painting with the black paint(remember LIGHT coats) apply all of the coats needed to completely paint it with primer or black, but never over paint.
after it dries, you can kick back and watch your masterpeice!