Knowing how to harmonize melodies is a crucial skill in songwriting or arranging music. Whether you want to harmonize the vocal melody in a pop song or the melody line in a rock guitar solo, the basic principles of harmonization are the same. As long as you know what key the melody is in, harmonization is an easy process. Harmonizing melodies can make song arrangements sound fuller and add extra extra intensity to specific parts of a melody.
Identify the key of the melody. You do not have to have a deep musical background to harmonize a melody, however you will have to at least know what key the melody is in to add a proper harmony.
Write out the notes of your melody. You can either perform this step on music manuscript paper or, if you cannot read music, simply write out the "letter names" of the notes on a regular sheet of paper.
Write out the notes of the scale. After determining the key of your melody, write out the note names of the scale degrees for the key on your paper. For example, if your melody is in the key of C, write: C D E F G A B.
Start at your first melody note and count up three notes. The most common harmony is called a "third." Harmonize each note in your melody up a third. For example, if your first note was C, you would harmonize with the note E. If your first note is F, you would harmonize with the note A. For the notes A and B, you simply beginning counting again from the left after you run out of notes. This means the note A would harmonize with C and B harmonizes with D.
Harmonize the remaining notes of the melody up a third. Follow the example used in Step 4 until all of the notes of your melody are harmonized.
Experiment. Once you have harmonized your melody in thirds, you can alter the harmonization to your liking. Although thirds is the most common harmony used in popular music, you can harmonize with any interval you like, as long as the note falls within the key. Try replacing a few of the harmonized notes with a fourth or fifth instead of a third. Harmonizing with fourths, fifths and sixths are the most common. Just use the same method that you learned in Step 4, but count up a different amount of notes. For a fourth, you will count up four notes. For a fifth, count up five notes.
Listen to songs with harmonies you like and try to deconstruct the harmonization. Examine if the harmony used all thirds or a combination of thirds and other intervals like fourths or fifths.
All of the examples mentioned have used "counting up" as the method for harmonizing melodies, as this is the most common technique found in popular music. This is because the vast majority of harmonies are written "higher" than the original melody. Harmonizing down can interfere with and muddy the original melody line, making the listener unsure which melody is meant to be the true focus of the song.