In each mounting bracket there are 2 slotted holes that mount the bumper to the brackets as well as 2 round holes that are only in the brackets themselves.
The slotted holes allow the bumper to be mounted with some adjust-ability, and can secure the bumper from moving around during small/moderate impacts. But if you really want to lock it down and make sure it wont move with winching and harder hits you will need to use a 3/8" bit and use the smaller round holes as a guide to drill through the bumper itself AFTER YOU HAVE THE BUMPER POSITIONED WHERE YOU WANT IT. Doing this will take all movement out of the bumper, so if its not adjusted or in its proper location that will just have to be where it stays once the holes are drilled.
Also, make sure that the side bolts on the bracket that secure them to the side of the frame are fully torqued, and this will help a great deal against the factory frame plates wanting to move.