There are many interconnected skills involved in organ playing, but the major ones that an organist would need can be reduced down to about 8 in number ... just like, in this photo of a heart jigsaw puzzle (above), there are 8 interconnected pieces which join to make a complete shape.
These 8 skills also interconnect with one another just like the pieces of a jigsaw puzzle and may be listed as follows:
1) organ manual and pedal technique (including touch and all the scales and arpeggios in their various combinations),
2) organ repertoire (including hymn playing, trios, and compositions from all national and historical periods),
3) sight reading (including C clefs and transposition),
4) improvisation (strict and free),
5) organ registration (stops, their possibilities singly and in combination, and national and historic schools of use),
6) harmony (including study of historical temperaments, the Circle, scales, chords, progressions, and other aspects of theory),
7) counterpoint (including invertible counterpoint and canon), and
These all join up to satisfy what an organist needs, and each one may be thought of as a piece to a jigsaw puzzle.
The first 4 puzzle pieces are learned mostly, if not totally, at the keys.
Much involving the last 4 can and has to be learned away from the keys.
Remembering the image in this photo ... and moreover, that it's a representation of where their own heart is ... can be very helpful to the serious student of the organ.
Anyone who's serious about learning it, whose heart is really in it, who wants to give some attention to each basic need that an organist has, will soon enough realize that unless they pay some heed to each one of these 8 puzzle pieces ... through careful, systematic, thoughtful, thorough, and gradual, slow as necessary, simultaneous study of each one over time ... they're own heart always will be conscious of, yearning for, and feeling the need for something that's missing.
Remember the jigsaw puzzle in your organ playing.