When and where should I modulate in a Bach Chorale harmonisation?

A really good way to help yourself with every step of Bach Chorales is to write out a chart of all the notes that the home key, and any keys that you modulate to. This will help make sure you can easily see what chords are possible. 

To work out when and where it modulates, play or sing the melody through a few times. See if you can notice anywhere where it sounds like the key changes -if it sounds like it modulates it probably does.

The other way to work out modulations is to look at cadence points. The cadence will probably only ever be a perfect cadence or an imperfect cadence. While Bach did use other kinds of cadences, you shouldn't need them at A level. From there, you can look at what keys it's possible to cadence in. There are a few places you can go but some are more likely to work than others. Bach often modulates to the dominant key, and also often modulates to the relative minor/major. Less commonly, he can modulate to the subdominant key, or to the tonic minor/major.

If you want it to modulate (or if the soprano line at the cadence doesn't work in the key you're in), think about what it's most likely to be: if it sounds like it's gone from the major to the minor, check if it works in the relative minor with either a perfect or imperfect cadence. If not, try the tonic minor instead. If it sounds like it's staying in the major, check if the cadence would work in the dominant, and if not, check the subdominant. If you really aren't sure, find a sensible modulation that the notes fit into and listen to it. If it sounds weird or wrong, try it another way as well. Taking a bit of time over modulations is definitely worth it, as they can shape your whole chorale.

From here, make your modulation. You want to have modulated well before the cadence. There are two basic ways to modulate: phrase modulation and pivot modulation. When harmonising a longer extract with a few modulations, try and include a few of both. Phrase modulations means that you modulate at the beginning of the phrase you are writing: eg, you end your first phrase in the home key and then start the next one in the relative major. The other kind is pivot modulations. To do this, you need to find a chord in the phrase (before the cadence point) where it works in both the home key and the one you are modulating to. Eg, if you are going to the key of D major from the key of G major, you might have a D major chord function as a pivot chord, as this is chord V of G major and chord I of D major. From here, use chords which affirm D major strongly. When doing a pivot modulation, try to modulate reasonably early in the phrase: you need the tonality to sound strong by the time you get to the cadence point. 

This can seem intimidating, but if you think about it step by step and do lots of practice chorales, you should be alright. As always with Bach Chorales, write out the chords you are planning to use before you write them in so that you don't get confused, and make sure you are constantly listening to what you've got.