Installing playfield mylar is quick and easy and will provide great results as long as you follow a few important steps. The two things that will ruin the job are dirt or air bubbles under the mylar. Although the process is fairly simple, there is some technique involved, so if it's your first time you might want to experiment on something besides your valuable playfield, like a piece of glass or mirror.
You can buy pre-cut mylar from several pinball sites or make your own (see the references).
Note: See the references for an article on static cling protective vinyl which unlike mylar is removable.
The first step is to clean the playfield where the mylar will be installed. Use an industrial strength orange cleaner which will remove wax and any residual glue if mylar was previously installed. Make sure the area is spotless and then wipe with a dry microfiber cloth. You can also use Goo-Gone to remove stubborn glue and clean the playfield with Novus if needed.
If you're installing mylar in a new location make sure all old wax is removed with orange cleaner, Naptha or denatured alcohol.
Fold back about 1/2" of the backing from one side of the mylar. This will allow you to place the mylar without it sticking to the playfield before you are ready. Carefully place the mylar into position.
If necessary you can use a toothpick or two to keep the exposed adhesive away from the playfield. Once you are satisfied with the placement use a bray (available at craft stores) or plastic roller (you can get a wallpaper seam roller at most hardware stores) to press down the first 1/2" of mylar (see Image 1). Do this working from the center towards each side.
Next, using a plastic or metal putty knife as shown in Image 1, press down on the mylar and then slide forward by alternately placing slight pressure on the left and right side. This should be done very slowly and if possible with a putty knife as wide as the mylar.
Initially you may have to use your other hand to peel the backing away from the mylar. Once you get going the backing will roll back away from the mylar as you move forward with the putty knife. Once the mylar is in place use a bray or plastic roller to roll out the mylar working from the inside out (see Image 2).
In Image 4 you can see the finished product.