Preferably such improvisation will be based on the exit hymn at the close of the church service. Here are a few of my recommendations (there are countless variations of such improvisations, but this is good for starters):
1. Place a hymn tune in the pedals.
2. Use easy figuration in sixteenth-notes which fits 5 fingers well: right hand descending (5321), left hand ascending (5321).
3. Play a short introduction of 2 measures in the hands on the first phrase of the tune.
4. Use a fixed harmony (chords) - 7th chords, 65 chords, 43 chords, 42 chords, for example.
5. Change the chords regularly (one chord for every phrase, for example).
6. When the pedals enter with the tune in equal note values, aim for step-wise motion in the hands - it's easier to control.
7. Remember to keep the same mode - then no matter what you play in your manual part will sound well with the pedals.
8. If you want to have more variety in color, change the mode every 4 measures or so - your tune doesn't have to stay in the same mode all the time.
9. Once you play the phrase of the tune in the pedals, repeat it in the manuals (without pedals).
10. End with a short CODA on a Tonic pedal point (the last note of the tune) repeating the last fragment several times in ascending transposing sequence (in major or minor 3rds).
11. Choose a loud registration with mixtures and reeds (if available) based on 16' stops.
BONUS TIP No. 1: Although it will be a fast piece, practice repeatedly REALLY slowly in fragments so that you are always in control and let your mind direct your fingers (and feet) and not otherwise.
BONUS TIP No. 2: Write down your improvisation on paper and see what can be improved. Having your own completed piece notated on staff notation is an incredible achievement in itself.
BONUS TIP No. 3: Actually you can start upside down and write your toccata on paper first and improvise later (it's more difficult but you will thank yourself later for doing so).
Remember to analyze real French toccatas (Boellmann, Widor, Vierne, Gigout, Durufle, Messiaen etc.). There you will find even more exciting figures, textures, harmonies, and models. But start small and only expand when you feel like you mastered the current version.
By the way, do 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.