how do you get the air out of it

For it to be most effective you have to pump away from the air scoop, this will lower the pressure allowing more air to be released from the water. This is based on Henry’s Law. Fluids under pressure will dissolve gases into it until the pressure is lowered, much like opening a bottle of soda or a deep sea diver ascending to the surface to quickly and getting the bends. 
I strongly recommend reading “Pumping Away” by Dan Holohan.