The A shape is the second movable barre chord. The root is on the A string, under your index finger: fret 2 is B, fret 3 is C, fret 5 is D.
Barre the index across the top five strings at the root fret. Then add the open A chord shape two frets higher: middle on D, ring on G, pinky on B. If that's too cramped, flatten your ring finger across the D, G and B strings instead. The low E is muted by the tip of your index finger touching it.
Pick through B major at the 2nd fret until every note rings. The high e is the one that usually dies; check that nothing leans on it.
Then combine both shapes in G major: G with the E shape at fret 3, C with the A shape at fret 3, D with the A shape at fret 5. Now three chords sit within a few frets of each other.
The most common mistake is strumming the low E on the A shape. Aim the strum at the A string and keep the index tip touching the low E.
