Get your empty two-liter bottle and fill it with the beverage you want to carbonate. The most important thing here is that your liquid is as cold as possible, because carbon dioxide is much more soluble in cold water than in warm. So chill your drink overnight in the fridge if you need to.
Once that puppy is cold, screw on the Carbonator Cap, connect the quick disconnect to the cap, and make sure everything is on there good and tight. Now, while the tank is connected, you've got to shake the shit out of your bottle. Shaking will increase the surface area between the gas and the liquid, which is where the transfer of CO2 happens. Shake it hard until you can't feel or hear any more gas being delivered to the bottle. This usually takes between thirty seconds and a minute.
Disconnect the Quick Disconnect valve and you're done. If you're only carbonating water, then you can unscrew the Carbonator Cap and you're ready to go. If you have anything with sugar in there (and yes, booze and fruit juice all have sugar in them) then you'll want to unscrew that cap really slowly so that it doesn't fizz up all over your counter.
And that's it! Now you can pour it out into glasses, or fill some bottles and cap them if that's the route you want to take. For reference, here's your shopping list.
CO2 tank: $68.40, $15 to refill.
Tap-Rite Regulator: $58.99
5 feet of hose and two hose clamps: $5.24
Screw-on hose barb: $0.95
Ball Valve Quick Disconnect: $9.99
Carbonator Cap: $12.65
Empty two liter bottle: $0.05
Total cost: $156.27