Every fingering that fits a hand
5320
easy
5656
moderate · barre
What is true of it
Cdim7 is C, E♭, G♭ and B♭♭ — 1, b3, b5 and bb7 counted from C.
B♭♭ is a double flat, and it has to be: every letter from C upwards is used once, so the bb7 cannot be written any other way.
The easiest of the 2 fingerings below is 5320: 3 fingers, held at the 2nd fret.
None of them uses an open string, so this chord moves up the neck unchanged — the same shape 1 fret higher is the chord a semitone above.
1 of the 2 need a barre.
Where its notes are on the neck
The brass dots are C, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
Cdim7/E♭
8989
Cdim7/G♭
bcbc
Cdim7/B♭♭
2323
An inversion is the same chord with a different note lowest. The name changes to a slash name because the bass is what a listener hears as the foundation.