Overflow boxes are designed to transport aquarium water from the inside to the outside.  You set your water height in your aquarium and then you set your skimmer box to be level with the water or just above like a 1/16".  Fill your skimmer box up with water and fill your overflow box with water.  If you have U-Tube type, place airline tubing inside all the way to the top of the U-Tube.  Place the U-Tube in your overflow box with the longer side in the outside box.  Water flows to it's lowest point is why the longer side goes to the back of the aquarium box Overflow) Suck out the air.  A little will remain, but don't worry about that.
           You should have your pump hooked up with a clear hose and a directional return sprayer.  I highly suggest that you purchase a check valve pvc type.  Some people think spring isn't good and swing is better.  They both have a spring in them.  They both flow the same and a PVC Check valve spring is going to be cheaper.  Check valves many times can screw in to a pump directly if you have threads on it.  Your directional return sprayer needs to be higher then the water level or it will drain your aquarium water level.  If you put a check valve on it, water won't drain from it. 
           Your ready to turn your pump on.  Water height in your aquarium will rise.  As it rises it goes in your skimmer box and begins to fill up.  Your U-Tubes are about 1" lower in the rear overflow box.  Water flows to the lowest point, so the U-Tubes automatically will flow when the water level rises in the skimmer box.  Your overflow box is set up now and running.  

I suggest you simulate a power failure and make sure all is working as it should.

Wet dry filters that are the skimmer box type are noisy and you have to have an airline pump pumping air out of the body.  They can also clog with debris since they sit at water height.

Sponge designed overflow boxes tend to be noisy like a toilet flushing every 5 seconds.  Sponges have to be cleaned often and replaced. 

If you get one that is a quiet overflow box it has a vent on it.  PVCFISH sells them.  These allow for air to be drawn in and prevents air vacuum pressure from occuring.

Cheers,
Tx Holey Rocks
Owner
Zak