This one is based simply on the fingering of a B7 chord. You start by going up the 5th string from open to your 2nd fret (B). You should do this in such a way that you end up with your middle finger on the 2nd fret of the 5th string, allowing you to assume a normal B7 fingering.
From there, we pick the 1st fret of the 4th string and then play the whole chord. Once again, simple but effective!