1.Put the car on flat ground, in park, or first gear with the ebrake engaged. Jack up the rear of the car with a floor jack or bottle jack using the rear jack points. Place jack stands or cribbing under the control arm. You will need the car high enough to fit the included screws in a socket wrench under the rocker panel. 

2. Identify the left and right skirts. The triangle pockets face up, and the narrow end is the front. 

3. Hold up the skirt to the bottom of the car. Prop up the skirt on cribbing, a cardboard box, or a second set of hands are needed to align them. Just set them in an initial position for now. 

4. Adjust the position. The triangle notch in the skirt is slightly oversized to allow for adjustment. 

Line up the front of the skirt with the edge of the OEM trim. Then slide the skirt inward until it stops. The triangle notch will set the amount of skirt that sticks out. 

5. Check that the back of the skirt is roughly lined up on the triangle notch. Slide a washer over a screw, and install the front screw. You can use a drill to start it, but do not tighten it yet.

A right angle drill attachment can make this easier so you don't need to jack up the car as high. 

6. Use a ratchet to finish tightening the screw. This will ensure you do not strip out the threads. This only needs to be tight enough to pull the skirt tight to the OEM trim. 

7. Move the object holding the skirt to the middle of the car to keep it from bowing. Align the skirt with the triangle notch, then slide inward until it stops. The skirt needs to still be flush on the bottom of the OEM trim. 

Then attach the rear screw in the skirt. Start with a drill, then use the ratchet to snug it up. 

8. Move to the middle, install a screw, and work out to each end installing each screw.