Yeah dude, they sell a kit at Walmart for less than $40.
Buy the kit, and follow the instructions. After you have the kit, you can buy more bottles, but you probably won't even need what the kit gives you for a regular recharge. I've done it to three cars, without removing any of the old stuff, and it works great. The R134 I believe is compatible with the old R12. Just don't breathe the shit, cuz it'll make you sick to your stomach.