Basically what he is saying is that if you have an overflow box rated at 600 gph make sure you pump is pushing just a little bit less than that. The oveflow will be 2 pieces one piece will be in the tank and the other piece will hang on the back of the tank, connecting the 2 will be a "U" shaped siphon tube. Water will flow into the box in the tank through the siphon tube to the box hanging on the back and down the drain pipe attached to the outside box. It will flow through your sump where your return pump will push it through PVC that you attached from the pump back up to the tank. Here are some pics of what mine looks like hopefully this helps.
Inside box.
Outside box and drain tube.
Return to tank.
