How do I harmonise a Bach chorale?

There is a small journey to make before you can get up to harmonising Bach Chorales. I would first recommend purchasing a (cheap) copy of Riemenschneider complete Bach chorales, and taking the time (2 min/day) to play through one or two on a piano. Before you start, have a listen to the chorales in the St Matthew Passion to get some idea of what they actually are, how they are used and what their harmonies sound like. We would then start by harmonising a simple hymn melody (i.e. 243 in the New English Hymnal) just to get some experience at 4 part writing. The most important line line is the bass line-this defines your harmonic trajectory. Start by writing the bass line for your cadences, and then fill out the passages in between. Try and make the bass line move in contrary motion where possible to the soprano line, and remember the prominence of primary chords! You should avoid chord 3 and 7 in this style. Once this is done, use either figures or Roman numerals to indicate what chord you are trying to outline, and fill in the alto and tenor sorts. This should still be kept as simple as possible, and try and maintain a stepwise motion in these lines. Check now for parallel 5ths and octaves, as well as any disjunct movement. You have the makings of a Bach chorale.