Guitar chords in the key of F:
F Gm Am Bb C Dm Edim

Chord Fdim7 notes: F, Cb, D and Ab. (Cb=B).

You must leave out the 2nd and 1st strings.

The string 4 (D) is left open.

Fdim7 - F diminished seventh guitar chord's alternative names: F°, F°7, Fo7, F7dim. See also: Fdim and Fm7b5.

Steps: 1-b3-b5-6(or bb7).
1(F), b3(G#/Ab), b5(B/Cb), 6(D).