about how much for 20% back window, how can you do it yourself

 

dont do it yourself. the back window is a @#$%, take it to a shop that will do it in one piece, not 2

 

Unless you do this everyday then please listen. There is an artform to doing this. You will waste your time and money in a bad way. I am sure tint would run $30 or so, and then figure in another $120 for labor.
Small price to pay for a great look

 

I agree with the others. Not tinting your windows correctly will make it look horrible, there's the issues of air pockets and streaking (not the fun kind like Will Ferrell did in Old School). A few years ago I had a local shop tint my '02 Grand Am with 20% tint. They charged me $200 for four windows and the rear windshield. The car was black so it looked awesome with them on. Even with it professionally done, there are some air pockets early which eventually even themselves out. Also, you can't open your windows for at least 48 hours after the tint has been applied (I waited a week just to be sure, I'm paranoid about the appearance).

Ultimately, just bite the bullet and let a shop do it for you. It will definitely save you time and aggravation, and also money since you'll most likely fail doing it yourself at first, then you'll need a professional to remove the bad tint and re-tint it.