Basically you run a strip of tape on the underside of the sideskirt. Just to the edge of the existing bolt holes. Then run a jigsaw along the tape line. (the tape gives you a straight edge to cut along. That is the only reason the tape is neccessary.) You have to trim the undeside because it is too long. You will hit the jack stand rails with the underside of the sideshirt, which will not allow you to place it on the car.
Now you are ready for the top part of the sideskirt. This line needs to be a straight as possible because it is the edge that will sit against the door lip on the car. Run another line of tape along the top. The closer to the rounded lip you make it, the closer it will sit to the car when you attach it. I left about 1/2 inch for mine.
test fit it on the car and hold it in place with jack stands. You will see just how much to cut. I attched it to the car with 3/4" self tapping screws. probably not recommended, but it has worked flawlessly for the last year. Fit it snug at the rear of the car because that side is pre-fabbed, For the front, trim that side if you are keeping your mudflaps. If not you have to make end caps.