Hi!
I don't use CO2 with my barrel. Basically, once you have finished fermenting (3 consecutive hydrometer readings showing NO change) you can syphon it into your keg. Now then - to carbonate your beer, make a syrup with 80-90g of sugar and add it to the keg first. Simply dissolve it in a little water - not too much as you don't want to dilute your brew too much, just enough to dissolve it all.
The yeast suspended in the syphoned beer will be enough to work on the sugar. CO2 will be produced during secondary fermentation. I leave my keg in a warm place for a few days while this is going on. After that, move the keg somewhere dark and cool to condition for as long as you can bear to wait! I think it is the cool phase that actually encourages absorption of CO2 into your beer.
If when you open the tap on your barrel, the beer comes out under decent pressure then you know it has all worked out.
I hope this helps!
