And so you start practicing it, you apply the same technique, the same practice procedures that you're used to in learning other pieces. However, you suddenly discover that technical demands of this piece are much more higher than you can currently achieve.
So what do you do in this situation?
You can switch to another organ piece which is easier for you or you can follow through and overcome these technical limitations and challenges and perfect the piece to the level when you will be ready for public performance.
The best way to follow through in a difficult organ piece is to subdivide it into manageable units. You see, if the piece is long and difficult when you first start playing it from the beginning until the end, you will be making many mistakes in each line.
Virtually every measure will have some mistakes in notes, rhythms, articulation and so on. But if you subdivide your difficult piece into fragments of about four measures long, then you can correct your mistakes on a higher degree.
This means that whenever you make a mistake, you can go back right away to the beginning of the fragment and fix it. It is best to choose a practice tempo in which you can avoid making mistakes which means playing at about 50% slower the concert tempo.
If you want to follow through in a difficult piece then you can also play these fragments in separate voices. You see, even though the fragment is short enough, you still may be making at least several mistakes, if you play all the parts and all the voices together right from the start.
Instead, you should work on practicing solo melodic lines and separate voices. Do this very slowly so in each repetition you will be playing without any errors. This technique works wonders, if you play each step and each combination at least three times in a row fluently (more if you want to be really good).
So do this in your challenging organ piece - slowly practice in fragments and solo parts and part combinations and you will discover how possible it is to overcome the challenges, follow through and perfect a difficult organ piece.