If you want to create a similar improvisation on any tune of your choice, here is the structure of this piece:
Verse 1: the Tonic key
Verse 2: the Dominant key
Verse 3: the Relative of the Dominant
Verse 4: the Relative key
Verse 5: the Relative of the Subdominant
Verse 6: the Subdominant key
Verse 7: Return to the home key
CODA (optional): modulation to the Subdominant key and a final cadence over a tonic pedal point. This creates a feeling of completion.
Choose a hymn, a chorale, or any kind of melody and try this simple approach for yourself. All you have to do is transpose your tune to these keys, choose a different registration for each verse and add a rhythmical figure for accompaniment based on chordal notes in the left hand part. This process is very simple but the beauty and impact of it is the fluency, different registrations, and change of keys for each verse.