Here are the instructions to start the syphon that came with my wet dry filter....
To start the syphon, you need to remove all the air from the U-Tube.
Take the tube out of the overflow box, insert an airline tube more than 1/2 way into the U-tube, and then reinstall the U-tube. With the U-tube in place, slowly pull the airline out until the end is at the highest point of the U-tube.
You need to fill the outside overflow box with water (the part of the outside box that the U-tube sits in), and then turn on the pump so that the tank water level rises and fills the prefilter box. Now suck on the airline tube to remove all the air from the U-tube. As you do this, the U-Tube will fill with water. Once all the air is removed from the U-tube and it is filled with water, pull the airline out. Your overflow box is now ready to go!