Forget the amateur stuff. For the best result, call a local carpet cleaning company with truck-mount equipment. Tell them you need your car upholstery cleaned. Get a competent guy with good equipment...the main thing is they need an upholstery tool and TRUCK MOUNT only. Cleaning car seats is no different than cleaning a couch and your local carpet cleaner will have all of the chemicals necessary to take ANYTHING out of your seat right away and far less hassle. You just pull up to their place, step back, and in 10 minutes (if that) the seats are like new.
I wouldn't charge more than $20 for two seats in a Ranger...and that's about a 75% profit. And they'd look brand new. Hell, throw in some protectant for an extra $5. I'm just saying, this is so common and so cheap, there's just no reason to mess with it yourself. Make friends with a carpet cleaner, man.
Seats are incredibly easy because of the padding behind them. Carpet, on the other hand, is impossible to do with normal extraction because the water just pools below since it's got no backing...if your carpet is stained badly, I recommend a new carpet off of eBay...they're under $150 and it's brand new carpet...no point in cleaning it if it's really filthy. Replacing it with rubber floor is just as cheap.