What Andre refers to is probably a verset (a short prelude of 1-2 minutes of duration.
Obviously chordal improvisation on soft sounding stops would work well here. In the above picture you can see one possible example. Along with the bass notes (played either with the left hand or with the pedals), the numbers above the notes indicate intervals to be played above the bass.
A sharp next to the number means that this interval should be sharp and the flat next to the number - that this note should be flat.
There is a modulation to G major in measure 15. Feel free to use it in your own improvisations.
What about you? How do you improvise preludes before the church service?