It is not easy, but in theory it is simple. First you must prepare the surface of the window, the window must be perfectly clean with not even the finest speck of dust. It saves a lot of time if you have proper tools for window tinting, for instance some hard-cards, a heat gun and some in razor blades.
It is also nice to have a tiny squeegee. You'll see seasoned professionals who also carry around paper towels and dishes soap. You also need to look out for the things like automotive caulking compounds and weather stripping sticking out, these can make for it to install, any need to take care these things upfront. Also, make sure there are no cellular phone or satellite radio antenna components on the inside the window. If there are you're going to the work around us any need to put extra liquid around us area so you can move the tent around to get in the right spot before you start cutting up the shape of the razor blade.
An average tint job could go between $75 and $150 depending on your vehicle, number of windows, type of tint and so forth...
If u want my advise... let a professional do it for you - don't get involved with something that seems very easy but could be a very hard thing to accomplish!