How do you start to harmonise a 4-part Bach-style Chorale?

Initially, decide upon the key in which the piece is written. Key signature, accidentals and the beginning and end of the melody are good indicators. Secondly, decide whether there are any modulations. If so, identify where they will take place. Thirdly, plan cadences, these are illustrated by the fermata's - pauses; over a given note. When cadences have been planned, the rest of the Chorale should adhere to convention ie: no parallel 5ths or octaves, voice leading should be easy to follow (no awkward intervals) and one should avoid doubling the third.