The secret is a long (about 15-20 foot) web strap. Wrap it around the bumper and run it through the car.
First check the bumper cover to see if anything is bent, in the way, or would otherwise prevent it from popping back into place. You’re going to have to wriggle under the car a bit. You might need some simple tools. Watch out for any hot parts like the muffler or the radiator.
I used it on the back bumper of a Mazda 3, and ran the strap through the back seat. Be sure to route the strap carefully to ensure it pulls the right way.
Once you have the strap in place snugly enough to stay without you tugging on it, check its position carefully one last time, then tug on it until the cover snaps back into place.
Is this the best way to fix your car? Probably not. Will it look okay when you finish? Probably. Will you save a lot of money? Most likely.