We highly recommend using double-sided 3M tape for the install. This will make your life much easier when drilling. In order to get a good adhesion, wipe the area with alcohol first.
Don't forget to also wipe the underbody where the tape will be, otherwise, the tape will not adhere to the dirty surface. You won't be able to clean it perfectly, but a little bit can go a long way in this case.
Here, Vince lays the double-sided tape on the cleaned surface. Pro-tip: don't peel the backside of the tape completely. Peel the backing of the long strips just enough so that they will fold under the diffuser while mounting. This will give you a little margin of error to adjust the diffuser into the perfect spot before it completely adheres to the under body. If you rip all of the tape off right away, you won't have this margin of error, forcing you to get it exactly perfect the first time in a “one-and-done” fashion. Don't worry, if you peel the backing around 3-4 inches, it will self curl downwards while mounting, and you will still be able to pull all of the backing out while it is in place.
Now the taped diffuser is ready to be fitted into place. This step usually helps to have three people total: one per each end of the diffuser and one person as a “back seat driver” to instruct the two mounters which direction to move the unit for a perfect fit while drinking a cold beverage and/or taking photos for the car owner's social media accounts. Not to worry if you are working solo- the cutouts for the rounded protrusions and the pinch weld make it incredibly easy to line up the fitment. We were anticipating more of a struggle for the first side, but after the first half of the installation, we left poor Vince to line up the second side alone. The equal fitment for both sides can be proof that it can be done alone!
Remember the pro tip about not ripping off the whole backside of the 3M tape? Here, you can see that the backing of the tape casually folds downward while mounting. You can begin pulling the strips of backing once you are satisfied with the mounting location. They pull easily, and then firmly press the diffuser to fully set the adhesion of the tape.