@Dagatris, You can do one of two ways. You can prime with sugar, yes, and then after you've consumed 1/2 of the beer, purge a little CO2 into the bottle (no quick disconnect, just open-end of the hose into the bottle, and gas a little), then seal the top and hit with Gas QD to pressurize.
Or, you can force carb directly in the bottle, no sugar needed. You figure out "when to stop" by calculating the Vols of CO2 in relation to the temperature you are holding the bottle at during carbonation, and the pressure from the CO2 tank you are applying. For example, for 2.2 vols carbonation in a beer being held in the fridge at 38*F, you would want to apply 8 PSI for a few days to a week to carbonate. If you wanted 2.8 vols instead, at the same temp, you would apply 14.3 PSI steadily.
(I won't delve too much further into all of the math, since it acts just like a corny keg or any other pressure vessel does. The stickies in the kegging section are a great reference for further info on this.)
But remember, you don't HAVE to open a 2L and drink half. It's the same size cap, so you could use 20oz bottles, 1L tonic water bottles, whatever size fits your needs best.
Also remember that you don't need one carbonator cap for every vessel. Once you are satisfied with the amount of carbonation in a particular bottle of beer, you can remove the carbonator cap and reapply a regular soda cap tightly. It will result in a very slight carb loss, but the liquid will quickly release enough CO2 to reach pressure equilibrium with the headspace, and all will be as normal. So, say you filled 5 2L bottles with British ESB, you could carb each one to a different level if you wanted to. You could examine the range of carbonation for that style by carbing one to 1.7 vols, one to 2.0 vols, one to 2.3 vols, and could even over-carb one intentionally to see how it affects the style.
Possibilities are endless.
