Then I'm sure you have most of the right tools. The only thing you'll likely need is a pentalobe screwdriver, and that you can get when you order the replacement screens. Tht will be to open a 4/4S/5 on the bottom. Inside is usually phillips.
Basically, what pm said is spot on.
The iPhone 3G/3GS/5 should be fairly easy to deal with, because the screen comes off the body first thing. Undo the screws, use a suction cup to pull the screen up and detach the cables. Pretty easy.
The 4/4S aren't HARD, it's just tedious. That's because the phone opens up from the rear an you've basically got to disassemble the whole thing to get to the screen.
I would imagine you're not going to be getting too many 2007 iPhones.