It's not as hard as it seems. You might want to get your rear window done professionally, but here's how you do it in a nutshell:
1) Get a spray bottle (windex or something) and put a little dish soap in it, and fill it up with water.
2) Get 2 pieces of tape, and separate the tint from the backing a little bit to determine which side the adhesive is facing.
3) Roll down the window about an inch, and spray down the outside of the window really well with the soap water, and roll out the tint on it, with the adhesive side facing out.
4) Using a razor blade, cut along the top of the window, and down the sides.
5) Roll the window up, and cut the rest of the sides, and then cut the bottom of the tint, to where you have about 1" of extra film below the bottom of the window
6) The piece of tint should match the window, except it has some extra on the bottom. Set it aside and thoroughly clean the inside of the window. Spray it with the soap water and use a razor blade to make sure nothing's stuck to the window.
7) Spray the side of your car with the soap water and stick the tint to it. Using a piece of tape on each side of a corner, like you did earlier, separate the tint from the backing, and get someone else to spray the tint as you're pulling it apart to keep it from sticking to itself.
8) Roll the window down down about 1", and spray it thoroughly, then stick the tint to the inside of the window. Spray the tint with plenty of the soap water. Line the top of the tint up with the top of the window. Using a squeegee, press down along the top of the window to remove any air bubbles, etc.
9) Roll the window up, and tuck the bottom into the gasket. After it is tucked, squeegee the hell out of the window. Keep it wet with the soap water-- the wetter, the better. Get all the water and bubbles out.
10) Wrap a 6" putty knife with a lint-free cloth, and press down firmly on the tint, and push any remaining water or air out from under the tint. Apply plenty of pressure.
11) ???
12) Profit