Where do you start with a Bach Chorale harmonisation?

Of course, there is more than one way to approach a Bach Chorale, but I think the first thing to do is identify the basic chords (I, V, IV, VI, II) in the key that you find yourself in. From there you have a armoury to employ. After this you need to identify candence points - find out which are open (that end on V), and which are closed (that end on I), and furthermore, whether we have moved away from the home key (potentially indicated by changes in accidentals). From here we can move to arranging a bass line, thinking about the standard progressions such as IIb-V-I or Ic-V-I for perfect cadences, and employ them at the cadences. Once this has been done, we can think about filling in the inner voices, being careful to avoid parallel fifths and octaves between parts. The hardest bit is fitting the inner parts to facilitate a modulation: start by completing the cadence of the new key, then work backwards bearing in mind the basic chords of the new key, and which chords can overlap (such as a if we move to the dominant the tonic of the previous key will be the subdominant of the new key) in a progression such as IV-V-I of the new key featuring chord I of the old key. From here it is a matter of fleshing out the rhythmic elements such as adding passing notes, often especially effective in the bass - however it is wise to look at the section the exam board have given you - if there is continuous quaver movement in this segment, they likely want you to continue this trend and vice verse if not.