it's not the most accurate way, but it works pretty well, as long as the o2 is working right. any voltmeter will work, but a digital will be easier to read. just make sure the car is warmed up before you make the adjustment. rev the motor to run some heat through the exhaust and double check the reading. o2 sensors like to be warm. if the motor is idling well, the reading will be somewhat stable, varying a few tenths of a volt. listen to the exhaust note back at the tailpipe. if you're getting some popping noises, you've gone too lean and the engine is misfiring. if it's too rich it will burn your eyes. the best way is to use a four gas, but most folks don't have one of those in the garage.