So you've got the breather on the top of your valve cover. When the engine is running, a bit of air and oil is gonna come out of there. Normally, the air and oil are dumped back into the intake system and burnt off in the engine. The point of these catch tanks is to provide another outlet, so the oil doesn't enter the engine.
So, the catch tank will have two nipples on it, and a petcock or other valve at the bottom. The petcock is for draining the oil that collects. You just drain the tank every time you change the oil, more or less. One of the nipples is for a line from the breather on the valve cover. The other nipple is for a vent, you can either put a small K&N right on the nipple, or run a line somewhere else (down below the engine) and put the filter there, so the fumes are venting away from everything else in the engine bay.
