Watch the Second Finger Guide online guitar lesson by Brad Carlton from Chord Studies: Slash Chords Vol. 1
This progression is a I-V/I-vi-V progression in the key of D. We start off with you're typical cowboy D chord, and to get our slash chord we lift our third finger and put down the second finger. That second finger will act as a guide when you move to the Bm7 as well as when you move back to the A7 chord. Feel free to experiment with different fingerings here.