Every fingering that fits a hand
x653
easy
b989
hard
b98x
easy
What is true of it
F♯dim is F♯, A and C — 1, b3 and b5 counted from F♯.
The easiest of the 3 fingerings below is x653: 3 fingers, held at the 3rd 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 string stays silent.
Where its notes are on the neck
The brass dots are F♯, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
F♯dim/A
2020
F♯dim/C
x020
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.