There are two methods for carbonating your brew:
Mix a measured amount of sugar with the beer and seal it tightly in bottles or a barrel. The sugar is consumed by the remaining yeast in the brew and the CO2 produced has nowhere to escape (no airlock) so it is absorbed by beer and thereby carbonates it. Natural carbonation is the method used when bottling and using a plastic pressure barrel. This method throws a sediment which is no good when using a cornykeg. The dip tube draws the beer from the very bottom of the keg so you’d be pulling sediment into your glass every time you poured yourself a pint. Some brewers opt for natural carbonation and snip the end of dip tube off. This is an option but it is a tricky balance between not drawing off sediment and not wasting too much beer. It is much more preferable to force carbonate when using these kegs.
This method gives the brewer more control over the level of carbonation, avoids sediment and is the preferred method when using cornykegs. Once your brew is in the cornykeg, pressurise the barrel to 30 psi or lbf/in2. It is a good idea to remove the tap and black disconnect for this, whereas the barrels can take up to 120psi, the taps cannot and may start to drip up at 30. Leave the psi at 30 and the CO2 will naturally drop into the brew over the next few days or a week. If you can’t wait that long, pressurise to 30 psi, lay the keg on its side, and gently roll it back and forth. This will increase the surface area of beer coming in contact with the CO2 encouraging a faster rate of absorption. It’s still worth leaving the keg somewhere cool for a few days for the CO2 to fully absorb to get the best results. Remember to occasionally check the pressure and add more CO2 if necessary. Store the beer at 10 psi and serve at 2-5 psi. The ideal psi may depend on the style of beer you're serving, for example one of the characteristics of a German wheat beer is it's high carbonation level, or may be a personal preference.