I started with the bottom bar and assembled it to the end brackets.

I used a hardware kit that you can buy to assemble the grille but decided against the pop-rivets because I don't have a pneumatic pop-rivet gun and I was concerned that if I painted myself into a corner it would be a lot easier to remove screws than it would to drill out rivets. Instead of the rivets, the grille is held together with #10 machine screws, one flat washer and a lock washer. They work just fine and can't be seen anyway.