With a soapy water solution (a tea spoon of dish soap for the whole spray bottle shown), spray down and wash the area. Make sure it's clean.

After you've cleaned the area, spray the soapy water liberally on the recessed area. Get it completely soaking wet with this dish soap water solution.

Lay the letter into the recessed area of the bumper. The soapy water solution will allow you to slide and maneuver the letter into place.

Take a step back and look at how the letter is mounted. Step back up and re-adjust the letter if needed. You have a few minutes so take your time. The letter will fit into the recessed area, so not much adjusting will be needed.

Carefully peel and apply each letter. Take your time. Continue to spray down the area with soapy water. Don't be afraid to really wet the area down.

If it's hot outside you will need to soak down each letter before you apply the insert for good measure.

Here you can see each letter fits neatly into the recessed area of the bumper.

Once everything is in place you can step back and check your work. Readjust any letters if you need to. Chances are that you won't, the letters fit into place easily.

If you notice any air bubbles, you can squeeze them out while the letters are drying.

Allow 24 hours for the letters to fully dry before blasting them with water, etc. You can drive your stang after the letters dry overnight. Hot weather or parking in the sun speeds up the drying time considerably.