Play a normal, first position G chord. Put your fourth finger on the third fret of the high E string, making a G. Put your third finget on the third fret of the B string, making a D. Instead of fingering the A string on the second fret like normal (which would make a B, which you don't want), use that finger to damp the A string. Or damp the A string by flopping your hand over just enough so that your second finger (which is playing a G on the third fret of the low E string) touches the A string.

John Peekstok