One of the challenges for new organists is finding time to practice on an organ. Most church organists do not own an organ, and need to travel to the church in order to practice, which can usually only happen once to a few times a week.
Practicing Without an Organ
Before I purchased my home organ, I did a lot of practicing on my piano and on my inexpensive electronic keyboard (which has full-sized keys). I was able to practice all of my manual technique on these keyboards. The sound was not the same, however I was still able to work on my attacks and releases, fingerings, etc. without having to go to the church and practice on the organ.
I would also sit at the piano, chair, or couch and visualize the pedalboard on my floor, then work out the pedaling and practice playing the pedals on my carpet. Mental practice can be very effective, especially when combined with the physical motions.
I would then play the piano while moving my feet as if I were at the organ. So much can be accomplished at home.
When to Practice
My favorite time to practice at the church is in the evenings when other activities are going on. I'm not alone in the building, but the chapel is almost always empty. I'm able to spend a couple of hours working on registration and technique, learning hymns, and finalizing everything for the service, without having to worry about unlocking the building or wondering who might show up when I'm there alone.
I've also been known to stay after church on Sundays and practice the organ (when it doesn't conflict with ward choir or another ward's meetings), or show up an hour or two before sacrament meeting begins to practice.
How Often to Practice
Ideally, I would spend two or more hours every day practicing the organ, so that I can work on hymns (including preludes, postludes, free harmonizations, etc.), "classical" pieces, and technique. Realistically, however, I strive for five hours a week at the minimum. You'll quickly learn how much time you'll need to practice in order to play with confidence and at the proper tempo for your worship service. Don't forget: do not underestimate the effectiveness of mental practicing!
Figure Out What Works for You
Ultimately, you will need to figure out what works the best for you. If you're up early, practicing in the morning makes a lot of sense. If your children are at school all day, practicing during the day works well. If you have young children or work full-time, practicing after they are in bed or after dinner might be the best decision for you.
Coming up with a practice schedule will help you make playing the organ a priority in your life. Such a schedule does not have to be rigid, but can serve as a guideline as you plan your week.