Watch the A Form (Alt. Fingering) online guitar lesson by Brad Carlton from Guitar Lab: Caging ii V I
The ii V I progression in the key of D consists of the following chords: Em (the ii chord), A (the V chord), and D (the I chord). This lesson uses the A form of the CAGED as home base with an alternate scale form. This will place you in sixth position. Example 7 shows the three chords in open position. Example 8 shows the D major scale in open position. Diagram 10 shows the arpeggio for the ii chord in red scale degrees. All numbering in all diagrams is oriented off the root of the chord. Therefore the notes: E, G, and B are the 1, b3, and 5 of the Em chord. The remaining scale tones are in black and are the source for harmonic and melodic enhancement. Remember that the scale you're playing is a D major scale but because you're playing it over the ii chord (Em), the scale is called E Dorian mode. In other words, one way to perceive the Dorian mode is to simply play a major scale starting and ending on the second step of the scale. The numbering that is presented in diagram one refers to the alterations of an E major scale necessary to form the Dorian mode.
Diagram 11 shows the arpeggio for the V chord in red and the remaining black tones formed the mixolydian mode. The mixolydian mode is the fifth mode of the major scale meaning you start and end on the fifth note. Again the numbers in diagram 11 refer to the alterations of an A major scale that produce an A mixolydian mode.
Diagram 12 shows the arpeggio for the I chord which is our home base. The red notes form the arpeggio in the black notes complete the entire major scale which is also called the Ionian mode.