Fuel Input: diesel goes to the Normally Open Position (NO) the Veg connects to the (NC) Normally Closed Position. The Output going to the IP goes to the Common Side

Fuel Return: The common side connects to the cigar hose that comes from the engine return. The (NO) Normally open side goes to the diesel tank. The (NC) Normally Closed side goes to the Veg Tank.

That is easy enough. But also you need to have two switches or one 3 position electric switch to flush the system so veg does not go into the diesel tank.

To do this with no power to the switch the diesel will go in and return goes to the diesel tank when you flip the switch on then the veg goes into the engine and the return goes to the veg tank. When you flush the Fuel in switch is off and diesel goes into the engine and the return switch is on and goes to the veg tank for a short period of time 15-30 seconds on average. Then both switches are turned off.

When you buy a switch get a 3 position with momentary hold. This means Middle Position is off. Up is on and Down has to be held down for it to work. That way it is really hard to flush all of the diesel into the veg tank. You are only flushing as long as you are holding the switch.