I used to do this a bunch in highschool.
First I made a template of the shape of the window with some wax paper, or something similar, so you can accurately cut the tint ahead of time. You'll want to over compensate for the size of the window just a bit, to allow for error. If the window will still be in the car and it rolls down add just a bit more to the bottom edge as well, maybe 2cm.
The trick for getting it not to crease, I found, is mixing some dish soap in with the water you spray on the window. Then use a squeegee (sp?) to smooth it out, you should have plenty of time to compensate for any mistakes or even remove it without too much hassle. Be sure to get the extra tint on the bottom below the sill, you can do this with a q-tip or popsicle stick, obviously the soapy water should be below the sill line as well. This edge doesn't need to be straight since no one will see it. Remember tint can tear easily and you don't want it hitting the sill every time you roll the window up or down. For windows that are not straight, instead of a squeegee, use a soft sponge, don't press to hard but it should make it form to the window a bit better. If you have problems with extremely curved windows, you might have to use less soapy water and go with nearly plain water, to get it to stick. If that is your situation, might be a bit of trial and error. Curved windows are the harder, but by no means undo-able. Air bubbles will not be tolerated.
When the tint is on the way you want it, using a razor cut a straight 5mm gap or so away from the edge of the window. Then with some clear nail polish seal the tint down by carefully going around the edge of the tint, no need to over do it. For windows that do not move there is usually a black lining around the outside, you should cut the tint just a bit over the black and apply the clear nail polish over the black. Having windows that roll up and down off the car make the process easier, and will be less likely tear around the sill. I never had the ambition to take out the window myself.
It wont be professional but unless you really look hard you wont be able to tell, and it can save you a whole bunch of money.