There are two possible fingerings for this note.
A flat sign is always placed before the note.
Like the sharp sign, the flat sign affects all B notes within the bar in which it appears. For example, in bars 1, 2 and 3 the b sign appears before the first B in each bar. The flat also applies to all other B notes within that particular bar. The effect of a flat sign is cancelled by a bar line, meaning that a new flat sign is needed to indicate a new Bb note in the following bar.