First, you will be required to carefully examine your paintball tanks. Perform an inspection on the tank to identify any cracks, leaks, scratches, nicks, gouges, burns or marks. If the tank is not safe to use, do not fill it with compressed air as this can be dangerous. If you do not know how to check the air tank, consider taking it to an expert who can help you decide if you should continue using it or buy a new one.
Once you are satisfied with the mechanical condition of the tank, load it in your car and take it to your nearest filling station. Connect the fill nipple of the tank with the fill station.
Find the shut off valve at the top end of the tank and make sure it is in the "off" position. You can use the booster to fill the tank or open the valves manually on the tank system. Fill the air to the pressure specified on the tank.
Continue to check the meter reading. When the pressure in the tank reaches the required value i.e 3000 psi or 4000 psi, simply close the shut off valve. Use the bleed off valve to remove any excess compressed air from the tank. Disconnect from tank's regulator at the filling station.