If a chord needs an accidental, a flat, sharp or natural sign is added as a figure.
It is either written next to the number it applies to, or on its own, depending on which note it applies to.
Accidentals will be needed if the harmony uses the raised 6th or 7th notes from the harmonic minor scale, or if the music changes key. Often, the final chord in a minor key piece will be a major tonic chord (Tierce de Picardie), and this will also require an accidental.
An accidental which appears next to a number applies to that particular note.
Here, “3#” means that the note which is a third higher than the bass needs to be sharpened. The chord needs an F#.
In this example, “6b” means that the note a 6th higher than the bass needs to be flattened, so the chord is Bb major in first inversion.
An accidental on its own (with no number next to it) always applies to the note which is a 3rd higher than the bass.
For example, this sharp is applied to the 3rd above the E, which means the chord needs G#.