Things You'll Need
Keg of beer with carbon dioxide tank
Beer tap line
Most beer tap systems are not very complex, consisting of little more than a pressurized tank, a hose and a spigot. They can require some troubleshooting at times, though. If large air pockets form in the beer line, or if the line is full of nothing but air when you start pouring, it can impede the flow of beer through the line and cause unpredictable spluttering and splashing, making it difficult to pour without foam getting everywhere. Fortunately, this is a relatively easy problem to solve.
Connect the tap line to the tap itself. Leave the line disconnected from the keg, or disconnect it. The line should be clean and ready for use at this point if you have not been pouring beer already. Make sure the line is securely attached to the tap and is undamaged so that air cannot leak in.
Place the cup underneath the tap to catch any liquid coming through it, and open the tap.
Raise the disconnected end of the tap line above the level of the tap and pour cold water into it. This will be easiest if you can lower the level of the tap and raise the line to a sink spigot; otherwise, use a large pitcher and funnel. Continue pouring water in until water starts flowing out the end of the tap, then close the tap and keep pouring the water in until the line is completely filled with water. Check to make sure there are no large air bubbles remaining in the line.
Connect the disconnected end of the line to the keg. Try to avoid spilling the water out of the line as much as possible while doing so, and make sure to attach it securely to avoid air leaks.
Connect the carbon dioxide tank to the keg. Open the valve on the tank and set it to the normal pressure level you use for dispensing the beer.
Open the tap and let all the water flow out. Do not open the tap all the way while you are doing this, as the tap will most likely sputter and may spray when the water ends and beer starts pouring out. Keeping the tap only partially open will help minimize splashing. Leave the tap open until only beer is flowing, then close it again. You will naturally waste a small amount of beer doing this, but the tap line will be free of air and ready for use.