Write out the melody line of your song. If you aren't sure about note placement on the music staff, refer to a staff line chart.
Decide which areas of the song you want to support with vocal harmony.
Isolate the notes in the areas that you chose and figure out notes that harmonize with the notes of the melody. You can do this by counting. Harmony notes within major chords (chords that have a "happy" sound) usually are four half steps above the melody note and seven half steps above the lead note. The half steps in music are A, A#/Bb, B, C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab. For instance, if the note in the melody is a C, an E or a G might sound good as harmony notes. If you're using a minor chord (chords with a "sad" sound), choose notes that are three half steps and seven half steps from the melody note.
Decide if the harmony notes that you chose should be higher or lower than the melody note. Notes that are higher should be written in on top of the melody line, and notes that are lower should be written in underneath the melody line. Traditionally, most harmony notes are below the melody line. For instance, if you are using a C in the melody and you want to add a G harmony note, write the G in the nearest G position on the staff lines directly underneath the C note. Again, consult a note placement chart if necessary.