You should have an auto switchover regulator between the two tanks. The red lever on the regulator points to one of the tanks. You open the valve on that tank first then open the valve on the other tank. The window in the center of the regulator shows green. The regulator now draws propane from the tank the red lever is pointing to. When that tank is empty the regulator automatically switches over to the other tank. When that happens, the window in the regulator shows red. You switch the little red lever over to the other tank, the window in the regulator not shows green again. You take the empty tank off and go get it filled. Hook it back up and you're good to go. Same process as above but now your drawing off the other tank with the recently filled tank standing by and ready to go for the next auto switchover.