Start by tuning open G with an electronic tuner. The notes for open G tuning from the bottom string to the top are: D G D G B D. When tuning using the above tab, always tune the measures in order from left to right or the tuning will be off.
Each measure has two notes. In the first measure, pluck the 7th and 12th fret harmonics so they sound at the same time. Then, turn the tuning peg on the 4th string until the two harmonics match.
Listen for beating (the wah-wah-wah sound). When the beating disappears, you are in tune. Follow the same procedure for measures 2 and 4. If you have a hard time hearing the harmonics in measure 4, try adding distortion.
Measures 3 and 5 each compare a harmonic with an open string. Once again, tune the second note of each pair until the two notes match.
Alternately, you can tune the classic rock open G tuning using a strobe tuner. To find out how to use a strobe tuner: