Select one of three types of harmony. In his article “Writing Great Vocal Harmonies,” Alan D Moore defines the three types of harmony. Write tight harmony by “following the melody line a third above or below.” If you want the harmony singer to sing one note of the chord until the chord changes, write chordal harmony. For a highly sophisticated sound, write contrapuntal harmony by creating a secondary melody. This secondary melody “is not necessarily the same as the main melody.” Experiment with the different harmony types by playing them on the piano.
Select your rhythmic complexity. If your melody is fast and has a lot of bounce to it, write the harmony with simpler, more sustained rhythms. Conversely, if the melody is slow and sustained, create a harmony part that is quicker and moves more. Write the harmony rhythm on the sheet music.
Apply the proper voice leading rules of music theory when writing your harmony on the sheet music. Avoid writing harmony that moves in parallel fourths, fifths and octaves. Maintain the melody notes above the harmony notes or vice versa; keep the two voices from crossing.