I see that the link I provided two years ago is longer good, so here's another one, from Autozone:



Duralast/Universal 3 port fuel selector valve used with SW48 toggle switch (FSV4) | Fuel Tank Selector Valve | AutoZone.com

As you can see, the valve has two input ports (one for each tank) and one output to the fuel pump. With the power to the valve turned off, it routes the fuel from one tank to the fuel pump (I chose this to be the main tank behind the seat.) But flip the switch on the dash and it energizes the solenoid and it now routes the fuel from the other tank (aux.) to the fuel pump. You can choose whichever tank you like as your primary tank. Be sure to use a power source that's keyed to the ignition switch, otherwise if you shut the engine off with the switch to the solenoid in the on position, power will still be going to the solenoid to hold it open for no need. I don't believe there's a lot of draw, but it could drain your battery over time if you leave it parked for a while.

It says the switch comes with instructions so I suppose all that's covered in there...