So what is the process for learning to improvise a chorale fantasia?
1. Take a hymn tune and create a two-part note-against-note counterpoint. The tune can be played in the top or the bottom voice. The most appropriate intervals for this step are major and minor thirds and sixths, perfect fifths and octaves. Avoid parallel fifths and octaves by using contrary motion between the voices as much as possible.
2. Harmonize a hymn tune in the treble clef only using the primary three-note chords and their inversions (the Tonic, the Subdominant, and the Dominant).
3. Harmonize a hymn tune in four parts (SATB) in the treble and the bass clef.
4. Enrich your harmonization with chords of the secondary three-note chords, their inversions, four-note chords, their inversions, tonicizations, and modulations.
5. Transpose your harmonizations into 5 closely related keys of the major or minor scale: the Dominant and its relative, the relative of the Tonic, the relative of the Subdominant and the Subdominant.
6. Repeat step 5 with the tune in the tenor (played with the solo registration on the different manual) and in the bass parts (with the reed in the pedals).
7. Repeat step 6 with the tune in half notes (the chords can change in quarter notes when appropriate).
8. Repeat step 7 adding non-chordal notes in eighth-notes, eighth-note triplets and sixteenth-notes.
9. Create a bicinium for two voices (the tune can be played in any of the voices with the solo registration).
10. Create a trio for three voices (the tune can be played in any of the voices with the solo registration).
11. Add imitative introduction and interludes in two and three voices between the chorale phrases. A single voice phrase can be used at the beginning.
12. Add diminutions in the voice that has the tune in four parts.
13. Add chordal echos for each of the chorale phrases.
14. Add melodic echos for each of the chorale phrases in three parts.
15. Add echo passages in sixteenth-notes for each of the chorale phrases in two parts.
16. Combine steps 9-14 to create a full-length fantasia.
So where to start?
Pick 10 hymn tunes that you like and practice step 1 on your instrument. Make sure you take a very slow tempo and don't advance to the next step with another set of 10 hymn tunes until you can play the current one at least 3 times in a row fluently.