My band uses capos a lot and it still messes with my head.....espically since we downtune
Basically, slap a capo on the indicated fret, play the chords indicated in their normal shapes. All the capo does is basically move the nut. So, if you play a G chord..... then slap a capo on the 2nd fret (two half steps), and play a G chord, now it's pretty much an A chord. Quick and easy key change without changing the color of the song. If you were playing bass, and learned the song in G, and the guitarists capo'd the second fret, you'd move the whole pattern up two half steps to start on A. Same pattern, different fret.