Amy
I wouldn't do it myself. Why? You would have to have royal skills in cutting thick plastic. Not only that, if you get it done professionally (like a window tinting business), it will look... professional. If you've never cut a decal before, why waste money trying to do it. Besides, the materials would cost you way more than just going to a business and getting them to do it for you. I have Spider-man and my name on the back of my car, both over 8 inches long and both of them cost me about $10 a piece. Look towards a window tinting place because they normally carry stuff like that.

acidcrap
One suggestion is just drawing the design up & bringing it to a shop that does custom decal work. This way they can just scan the design and laser cut it out for you. A lot less hassles than trying to cut the stuff yourself. It's still going to be a custom decal that you designed. Your just not doing the labor.

toonew2two
I suppose it depends on how big you want this thing... the other two answers seem to be headed for something really big that covers you entire window and then I would have to agree with them. Get it professionally done. BUT if you want it like hand sized or paper sized or so then there are paints (at any major craft store and I think Walmart) with which you can paint on to clear velum and then transfer it to your car window. *advantage* you can trace through that velum and get a really good drawing *disadvantage* I don't know how the stuff holds up in weather. It is kind of meant to fake stained glass. How ever if you are making it yourself...