It's pretty simple if you keep in mind two simple concepts: one, gravity, and the other that you have to fill the entire thing with water, from end to end, and not let air interrupt it.
Like people have said... fillup the barrel with water. Tilt up the barrel and let gravity make the water flow down the tube. When it reaches the end, put your finger over it. Now you have water throughout the whole system. Resubmerge the barrel and let it fill up again with water until no more air bubbles come out. Release the end. Voila. Don't interrupt the flow, and your good to go. *jk*
(This title was quite disgusting especially during breakfast.)