The Fdim has a muted string right in the middle which can be a little difficult at first. The index finger is placed on the first fret of the G string. Next the middle finger goes on the first fret of the E string and the ring finger is on the second fret of the A string. Normally we don’t want our fingers touching the wrong strings but here we can let the index lightly rest on the C string. Play every string to make sure the ones that are supposed to sound are clear and that the muted string stays dead.