The Dorian Mode is based on the second note of the Major Scale. In the key of G the second note of the scale is A, therefore this would be played as the A Dorian Mode. To play this mode in the key of G, begin by playing the first note at the 6th string 5th fret A. Notice that we are in the key of G, so I have identified the roots (G's) in white and the perfect 5ths (D's) in red for all of the modes.