How do you switch from main gas tank to aux gas tank in 72 Chevy trucks?

If your referring to light duty C/K 10/20 pickups with factory auxiliary tank(s) there is a valve mounted on the floor between the door and seat on the drivers side that just needs a twist. There is also a knob on the dashboard to the left of the instrument cluster that will switch the fuel gauge from showing the amount of fuel in the main and auxiliary tanks