The standard grips this chord is usually shown with.
F#dim is F#, A and C, a diminished triad. The shape this page opens on is written 20x21x, lowest string first.

Also written F#°, F#o or F# diminished. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 5 common shapes for F#dim on the guitar, and 53 derived voicings in 2 families.
In which keys. Every note of F#dim belongs to E minor (the ii°) and G major (the vii°).
The common shapes, written lowest string first: 20x21x, xx45x5, x97x78, x-9-10-11-10-x, 14-12-x-14-13-x.
Triad. Three notes across one string set. F#dim has xxx212, x342xx, 2342xx, 534xxx, 5345xx, x045xx, xx757x, xxx575.
Open string. An open string left ringing against a fretted grip. Because it costs no finger, it sounds a note the closed shape has none left for -- a tension on top, or a pedal root underneath. Either a familiar grip with open strings added, or a shape so sparse the open strings carry the chord. These belong to their key rather than to a shape, so D, A, G and E have many and Eb a handful. F#dim has 23420x, x00575, 53450x, 20050x, x00502, 5x050x.
Tensions. A shell of F#dim, root, third and seventh, leaves a string free, and the tones that belong over it are the 11 (B) and the b13 (D).
The common shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.