You're not going to need anything to apply the piece. Basically just locate the notch on either side of the bumper lip down there and start pressing it on. It's not real easy to get it to seat, but you can definitely tell when it's seated all the way. Take a look under there and make sure the lip is in the back part of the groove if you're not sure. Then you can work down it and squeeze it all on. You'll feel a click when it seats the bumper lip into the groove, then just make sure 100% of it is mated.

Many members have bought a replacement and lost it soon thereafter, so the chances of it flying off if you don't seat it are high. If it's really difficult to seat you probably did it right. If it was too easy, maybe take it off and use some pliers to decrease the gap.