Your melody track (regardless of which channel) must have every note strike a pitch adjacent to the previous one -- i.e. each note must be exactly 1 or 2 semitones above or below the previous note.
You can use any key signature, and you may use accidentals (like an F natural in G major) if you need them.
As an example of some possibilities, see the attached file for two melodies I wrote based on this rule.
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"im going to continue making this crazy stuff then after a while my style will be so sick that you will be like damn suuun that shit is so sick i dont even get it. i will be like bro its ok.. you dont have to." -omgdonut
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