Many here are more knowledgeable that I, but here's what I have done for the past 6 kegs with good success. Cold. With cold keg of beer I attach CO2 and turn pressure up to 30. I lay the keg on its side with CO2 line to the bottom and roll back and forth gently or not so gently...listening to the nice gurgling of the CO2 going into the keg as more CO2 goes into solution. Roll more, wait, roll more, wait, roll more, wait... I continue this process for the better part of an hour or so...probably rocking and rolling a couple of dozen times. Back off of the pressure, bleed excess from the keg and there you go. If I don't need beer right away, I leave the pressure up around 10 for a couple more days. Good luck.