Personally I would go to the Hardware store and get a bolt/nut combo whatever size I think would be best. Nothing huge. There are already holes in the bumper. I would pop off the front black grill, just push it pushes right in. Then I would drill holes in the lip to match the existing holes in the bumper. Then you should be good to install.
edit: well i guess the grill thing is on my FRS sorry just noticed you had BRZ. Check to see if grill comes out the same way so you dont break anything
