Before practicing your organ piece you have to decide whether you will be playing it from music score or from memory at that public performance. Some people like challenges, like playing from memory in public. Some people are afraid of playing in public from memory and get a high degree of performance anxiety. They are afraid of losing their memory in technically challenging places.
So you have to decide for yourself whether you will be playing a piece from memory or from the score. Keeping that in mind, I still recommend you memorize it. It doesn't mean you have to perform it from memory in public but the memorization process will be for your own benefit and fluency of this piece.
So how do you memorize an organ piece? Although there are at least several music memorization systems, I like the following one.
Subdivide your piece into fragments of four measures and memorize each individual measure in that fragment. This usually takes about 15 minutes of practice. The way you do this is as follows.
Play measure one five times from the score and five times without the score. Do the same for measures two, three, and four. After that memorize two measures at a time: one and two, two and three, and three and four.
The next step would be to practice three measures at a time: one, two, three and two, three, four. Finally, you can memorize all four measures the same way. Again, play five times from the score and five times without looking at the score.
Can you see where I'm going with this method?
The next step would be to memorize the next fragment. Do this for each individual fragment of your piece. After you reach the end of the piece, you have to go back and memorize two fragments in the row.
That will be eight measures. Later you will be playing four fragments combined and so on. With every step you will make your fragments longer and longer. Remember that at the beginning of each practice session you have to repeat previously mastered material before learning something new.
Try to apply these tips for memorization in your practice and you will be ready for public performance of your organ piece.