You can plan out the chords and the structure before you write the melody.
Write out all the chords you can use:
Write in the chords you want for each bar (each count of four):
| 4 4 | |||
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Choose a different set of chords for the B section.
| 4 4 | |||
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Decide what the order of the sections will be. (Remember ternary form…AABA or verse chorus verse chorus...).
Write out the structure:
AABA (for example)
Now you need to put a tune on top.
Either use a flute to play a melody (you can write the lyrics underneath the tune on the score) or sing and record your melody over the top.