Key: D Major
I ii IV I
I: D Major ii: e minor IV: G Major I: D Major
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One easy way to make melodies from a chord progression is to use chord tones. Using your example, an
I ii IV I progression in D would make use of these chords:
D e G D
Whose notes are:
(D, #F, A) (E, G, B) (G, B, D)
You could just write a melody picking from the tones from the chord you're writing over. Using this example:
To further spice the melody up, you can vary the rhythm and add melodic decorations such as passing tones, anticipation notes, and auxiliary notes.
This can be done with any chord progression. For more about melodic decorations, see this website.