Find the circuit breaker in the electrical panel for the well pump and pressure switch. Turn it off.
Open a water valve closest to the pressurized water tank and drain all the water out of the tank.
Confirm the tank is not waterlogged before continuing. If it is waterlogged, it will require a complete tank replacement. Jiggle the tank back and forth carefully -- avoid breaking the water line connections when you do this -- and listen for sounds of water. If the tank still retains water, the rubber bladder in the tank is broken. The only solution is to replace the tank, as the rubber bladder cannot be fixed inside a sealed unit.
Place the tire gauge on the tank's air fill valve and check the pressure. The air pressure in the tank should be 2 pounds per square inch below the cut-in setting on the pressure switch. For instance, if the cut-in setting is set at 30 psi, then the pressure tanks needs 28 psi when empty. Add air to the tank using the portable air tank and check the air setting again. Adjust as necessary by depressing the small stem inside the air fill valve to release extra air.
Close the water valve and turn the pump back on. Allow the pressure tank to fill with water. When full, the system is fully operational.