Theme - in two voices (the tune in soprano)
Var.1: In two voices (the tune in soprano ornamented in running 16ths)
Var. 2: In three voices (the tune in soprano, the bass in 16ths)
Var. 3: In three voices (the tune in soprano in 8th note triplets)
Var. 4: In three voices (the tune in the soprano, the bass in 8th note triplets arpeggio)
Var. 5: In two voices (the tune in the soprano with imitation between the hands in 8th notes)
Var. 6: Running 16ths in the bass, chords in the right hand (tune in soprano)
Var. 7: Scale passages in the soprano, use the bass which enforces the harmony of the tune.
Var. 8: Change of mode: tune in the soprano. Movement in 8th notes, some imitation between the voices.
Var. 9: Original mode: three voices, movement in quarter notes with some imitation.
Var. 10: 16th note arpeggio figure, tune in soprano alternating with the bass.
Var. 11: Free variation which only has the same harmonies as the theme - dotted rhythms, virtuoso running 32ths.
Var 12: Change in meter, theme in soprano, running bass in 16th notes. Later both hands are in 16ths.
Coda (optional) - you can take a distant key, only a fragment of the theme (one motive) and build entire variation out of it. At the end add a sudden enharmonic modulation to the home key. End your variations with a bang.
Follow this procedure when creating your variations on the organ today. After you can play them for yourself, be sure to play it with your friends and family. If it is really successful, you can even record a video of your piece and put it on YouTube.
It is essential that you look at the actual pieces by Mozart and see how he has created them. By analyzing how the pieces are put together you can follow his examples in your compositions or improvisations and in doing so Mozart's musical language will become your own.
Before improvising such pieces it is best that you compose at least 10 similar ones on paper. Don't forget the benefit of memorizing your model variations of Mozart and transposing it into different keys.
By the way, do you want to learn to improvise in the style of Bach? If so, I suggest you check out my 9 day mini course in Keyboard Prelude Improvisation.
Or if you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.