Open a polyphonic organ composition that you are working on right now (it could be any chorale prelude, fugue, ricercar, chorale fantasia etc.). Remember, one of the first steps to take, if you want to master this piece is to practice in separate voices (soprano, alto, tenor, or bass).
But instead of playing them on the organ, try to sing them. You can use solfege syllables but if you are not comfortable using do, re, mi and so on, you can sing using neutral syllables, such as la, la, la.
Through this method of practice you are not only getting to know each line on a deeper level but you are also training your ear. It is best if you don’t play the line which you are supposed to sing at the same time.
To make singing without an instrument easier, you can imagine the scale degrees of the particular key you are currently in. You can even write them with pencil on the score.
To take this practice one step further, you can sing one voice, but play another and vice versa. Later on add another voice until you can play three voices and sing any fourth one in a four-part piece.
Start applying these tips in your organ practice today. In just two weeks, you will begin to discover some tremendous changes in your ear training skills and analytical abilities which will help you advance as an organist to the next level.
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.