In 171124 MMDigest David Smith said, "I am the (proud) owner of
a MIDI-controlled, 31-note Rob Barker hand-turned busker organ.
I am quite musically able and would like to produce some music files
for the instrument for myself... I have little or no knowledge of
how to go about it.  Is there a publication that would help me..."

Start here:
http://wallace-venable.name/mechanical-music/Princeton-Arranging/21st_Century_Arranging_for_Band_Organs.html 

He added, "Or can any one give me blow-by-blow assistance?"

Perhaps we can come to blows after you have read about the basics.
I certainly will try to answer specific questions.

Wallace Venable