Other than a vertical doors conversion kit, an auto body kit is one of the most exciting upgrades you can do to your vehicle. A good aero kit or body kit can really transform the appearance of the vehicle, and the installer will determine if this is a good or bad transformation.
I would personally suggest you find a reputable shop experienced in your type of body kit work. They should know how to work with the material you selected, and how to color match, prep and paint the kit. They should also know how to, if necessary, make adjustments to ensure a proper fitting. Your finished product will be as good as the effort put into it.
Having said that, it is never a bad idea to learn something new. Before you begin, make sure you designate enough time to the project to install your body kit right the first time. This type of body work requires a lot of patience if you are inexperienced at it. Here are some instructions to help you get started on the project.
The first thing to do is to prep the body kit for painting. It will be much easier to work on before it is installed. If you do not intend to paint the whole vehicle, it may be wise to color match and paint the body kit prior to installation.
Start by removing the pieces that will be replaced. This may include the front and rear bumper covers. You can find the bolts for the front cover in the fender wells and underneath the cover near the frame. Remove these bolts and loosen the cover. Depending on the application, be sure to remove any of the lighting that is attached to it, such as the headlights, side markers, fog lights, etc. Now check to make sure that there isn't anything else holding it in place, such as a grille or license plate holder. Carefully remove the front bumper cover and set to the side. Keep the hardware handy, as you will need it later. Remove the rear bumper in the same manner, as well as the trim panel located just above the bumper (if applicable). Most cars won't have side skirts at this point, but if it does, remove them by removing the bolts that hold it to the vehicle. They are located on the back of the side skirts.
Take your replacement cover and carefully align it to the car and check the fitting. Some of the kits will have 3M adhesive to help hold the kit in place during this process. You may find that the foam inserts are an obstruction to the new body kit. You can either remove them or trim them to size, depending on the application. I have found that if possible, it is better to trim them to fit because they will help sturdy the body kit so it doesn't appear as flimsy. If you have done your research and purchased a good kit, then you should line up almost perfect. Make some markings as to where you will drill your holes. You can also use the old bumper cover to line up the holes but it may not be as accurate as the fitting approach.
If you find yourself in a situation where the kit doesn't line up exactly, there are some things you can do to correct the problem. First, locate the problem areas. They will be most noticeable on the sides where it matches up to the fenders or in the very front where it matches up to the hood. If you notice one side has a bigger seam in these areas than the other side, try moving the cover to the larger seam side in very small increments until there is no longer a difference. The seam should be the same width as other seams on the car, most notably the same size as the seam from the fender to the door, or the fender to the closed hood. Using these same widths for the seams will create a "seamless" look for the finished product.
If your problem area is in the front, you will have to adjust the body kit in small increments until it lines up. If you have too larges of a seam, try gently pulling the kit from the two sides to tighten up the front. it should have a little bit of play in it since it is not bolted down. If the seam is to small, try pushing the sides in towards the front of the vehicle to free up any space. Once the front is adjusted, look to see if the sides are aligned property. If you are happy with the fitment, place the bolts in the holes and tighten them down. Start in the middle, and work your way out to the sides making sure that your seams line up properly. If your kit comes with 3M adhesive on the back side, adhere the kit to the car once the proper alignment is made. This will help hold the kit in place and allow the seams to be smaller in width.
In some unfair and unfortunate circumstances, you may find your kits not lining up after all these adjustments. These kits will require some body work or fiberglass work to be done for proper fitting, such as trimming or sanding up the side edges and refinishing them, or repositioning the bolts. Hopefully you won't encounter any of these types of problems, because they tend to have to be solved on a case by case basis. If you find yourself in dire straights, contact firstname.lastname@example.org and we can help guide you to the best solution.
With your front piece of the body kit installed, you can concentrate on the side skits and rear bumper cover. The side skirts should be fairly simple, and will require drilling pilot holes in the bottom of the car. To ensure proper alignment here, attach the side skirt to the vehicle using the 3M adhesive (if provided). It should line up exactly with a small seam in the front and rear next to the fenders. If the seam is larger on one side, split the difference and drill the pilot holes for the bolts and secure it in place.
Now move to the rear bumper cover. Follow the same procedures as above until you get a goof fitting. Again, you may need to remove or trim the rear bumper foam inserts, depending on the shape of the body kit. Secure it in place and enjoy the finished product.