You should not have to change either the plugs or the oil. If you are changing the oil and or the plugs anyway because they are due, change after the treatment.
Either the aerosol method or the liquid method work fine. I think you can get a bit more material into the system faster with the liquid so it may clean a bit better, but I doubt that the difference is all that much. I've done it both ways, and i kind of like the liquid way better, but the aorosol method is easier.
You really should clean the throttle body at the same time as well as add a can of injector cleaner to the fuel tank to finish cleaning the combustion chambers as well as the injectors.
