That's good advice from kkt.
Here are a few more tips.
1. First clean out any small rocks, debris from the base of the radiator. There is a small "channel" where the radiator and bottom cap meet that gets full of gravel. One person I know found a dead bird in there (I'm serious). You can access this area with the bumper on. Just lay on your belly.
2. Remove the spare tire also. With it out of the way, it will be easier to move the radiator back and forth to get the shield down in there.
3. You can move the radiator back and forth quite a bit, but as the old rule from junior high wood shop goes "don't force it".
4. The compression ring goes on top. Install it before getting started / before pushing the shield in.
5. It shield should slide down enough for you to get the radiator brackets back on. Sometimes (it varies from car to car) the shield doesn't slide far down enough and sticks up. If that happens, you have to trim out about a 2" x 2" square out of the bottom corners of the shield so that it will slide down past some plastic parts that are down there. If you have to do that, don't worry, there is still plenty of coverage for the radiators due to the plastic bits that were blocking the shield. Hopefully you have some tin snips in case you have to go this route.