Play the manual parts on piano or keyboard while you sing the pedal part (fixed-do solfege can be useful here) and move your feet on an imaginary pedal board.
I have learned relatively easy pieces this way and performed them without having had much time with an actual organ.
Even for a Bach fugue, though, if you can sing the pedal part while playing the hands it will be much easier to add the actual foot action.
music.stackexchange.com/users/17187/musarithmia
Don't know where you live, but here in U.K. the old style of organ (2 manuals, pedals), come along on occasions for little money.
I bought one for £10 sterling, just for the pedalboard (although the 2 12" speakers were a nice bonus).
If you can find room, this is a good option, as pretending won't tell you when you've hit a wrong note.
There may only be one octave of pedals, rather than two, and they'll be parallel, not radial, but that's got to be better than none.
To practise an instrument and improve, every day is the way to go.
Even if it goes in the garage, bedroom, whatever, it's better than the once a week you're doing.
music.stackexchange.com/users/2808/Tim
I find that moving my feet as if there were a pedal board, when practicing on a piano, for example, helps.
It engages the learning centre of the brain - not as much as if you had the real thing, but better than not.
Playing the Organ is hard - it will take a long time to get proficiant.
Have you got pedal exercises to do?
working through them will teach your legs different pedaling techniques, and increase your accuracy.
Are there any other churches/organs in the area that you can get some practice at?
Many churches are more than happy to let people practice on the organ.
music.stackexchange.com/users/9396/7thGalaxy
The cheapest solution.
Take a cardboard.
Draw the keys on it (of the original size).
At least, You can be confident with coordination.
I used this method (however, for hands), when I needed practicing piano pieces written for the grand piano on my upright piano (which doesn't have some keys at the top).
That's enough for studying a piece on the first stages, but anyway You have to practice in the real environment, especially the closer the concert / competition is to take place.
Actually that is sufficient even to become a laureate of the competition, if You really desire (written from my personal experience).
Just a side note: Sergei Rachmaninoff used "silent" keyboard ("немая" клавиатура) during traveling.
The Cardboard Piano : Perkins's "cardboard piano" derives from an incident (...) about the celebrated composer Sergei Rachmaninoff who practiced on a silent piano while crossing the Atlantic.
music.stackexchange.com/users/9553/Dmytro Dzyubak
buy a flexible midi pedal board from china.
It can be folded and you do not need space.
Just spread it on the floor when needed and connect it .
You will need a midi adapter to usb and of course a computer with sound output connected to speakers.
Definately midi software.lots out there.
music.stackexchange.com/users/24297/don
I would often just picture the foot pedals in my mind and move my feet to an approximate point on the floor.
The mental effort of coordinating the foot movement along with everything else is worth it even if you don't have access to a real foot pedal.
Keep in mind that pedal distances are not the same from one organ to the next even if they're the same type (radial vs parallel)
so the "muscle memory" required is different than for the hands.
Eventually, you should get to a point where you can sit at any organ and spend no more than 5 minutes getting familiar with the foot pedals then you're good to go.
music.stackexchange.com/users/171/chabzjo
I only get about an hour-and-a-half's access per week to practice on a 'real' organ.   Needless to say, my progress at pedalling is painfully slow.  Are there any ways to practice pedalling without the pedals (I used to practice touch-typing by shutting my eyes and just imaging)? And if not, is there a way to make my practice sessions more productive?
Practicing pedalling without pedals, or maximizing real practice efficiency
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<organ><keyboard-pedals>
music.stackexchange.com/questions/15936
music.stackexchange.com/users/9796/Benjol
 Moving your feet as if there were a pedal board, when practicing on a piano may help with organ pedal practice. Alternatively,  buy a flexible midi pedal board from China. To practise without a real organ, you could move your feet to play imaginary pedals whilst singing the pedal part. Alternatively, you could consider buying a flexible MIDI pedal board from China.
Moving your feet to play imaginary pedals whilst singing the pedal part is considered a good practice technique when you don't have access to a real organ. You could consider buying a flexible MIDI pedal board from China.