1. Notate the melody of the song in a stave notation. You can use a recording as your foundation for notating or sheet music of the original song.
2. Harmonize the song in 4 parts keeping the tune in soprano. Keep the original harmonies and chords or add your own harmonization. In order to be able to harmonize a tune, you will have to know the basic rules of harmony and voice-leading.
3. Add figuration in the accompaniment for more variety. The figuration can be melodic, rhythmic or both. A good example of figuration could be arpeggio, scalar passages based on chords. You can also add non chordal tones, such as diatonic or chromatic passing tones, neighbor tones, appogiaturas etc.
4. You can put the tune in the tenor of your harmonization. An interesting arrangement will have the melody of the song in the tenor voice played by the left hand while the right hand takes soprano and alto. The bass will be played by the pedals.
5. Notate the introduction, interludes, and conclusion. You can do this step either from the original or creating your own. Your arrangement will be incomplete without these structural parts of the composition. Again, use the original harmonies or create your own.
6. Write in the registration. Consider this point carefully and think which stops can be most useful in bringing out the tune? If the tune is in the right hand, softer reeds, compound stops and mutation combinations will sound very well. If the tune is in the left hand, play it on a reed. Use softer stops, such as flutes for the accompaniment.
7. Add dynamics. Once your harmonization, figuration, introduction, interludes, and endings are complete, notate the proper dynamics. Think about the culmination point of your song which often is the loudest part of the piece.
8. Add phrasing. Your arrangement also has to have phrasing markings. These are lines which connect several measures into one phrase. Usually a sign for the ending of the phrase is a breath mark (caesura), a rest, a long note, repetition of rhythmical figures, change in texture, change in dynamics etc.
Use the above steps to create your own unique and original organ arrangement of any song of your choice today. You will definitely have fun and learn many new things in the process. Once the arrangement is complete, you can impress your friends or family by learning to play it on the organ.
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