Start GarageBand and create a new music project.
(You may skip to step three if you wish to harmonize an existing musical line. )
Click or double-click the GarageBand icon, which you’ll find in the Applications folder, or perhaps on your Dock. You can also use Spotlight (command-space, or the magnifying glass in the top-right of your screen) to find it.
Click File, then New to start a new music project.
Record the melody.
Use an external MIDI keyboard if you have one, or click Window/Musical Typing.
Rehearse playing the first bar of “Mary had a Little Lamb” using the D, S and A keys on the computer keyboard for notes E, D, and C, respectively. You can use another melody if you desire.
Press record--the round button to the left of the playback controls—to start recording. Press the spacebar when you’re done.
Copy the melody.
Double-click the melody you’ve just created, which will appear as a series of short lines within a colored, rounded-corner Garageband “region.” This will slide up a grid-lined edit panel from the bottom of the screen, in which you’ll see the melodic line you’ve just created on the grid.
Melody is the new harmony
Press command-A again, if necessary, to make sure all the notes are highlighted. Click and drag the melody to the desired harmony location--up four half steps, or a major third, is a common harmonic interval. Since “Mary had a Little Lamb,” starts on the third-scale degree, use three half steps, so your beginning note is now G.
Paste the melody back in.
Click and drag the triangle above the red play line to move it to the beginning of the measure. Press control-V to paste in your original melody.
Adjust individual harmony pitches.
Drag the play line back to the beginning and play the melody and harmony back. Stop as soon as you hear a pitch that doesn’t sound good—music rules are not perfectly linear. In this example, the third note in the harmony needs to come up half a step. Click and drag the note to correct it. Play back to verify. Repeat for other pitches in your harmonic line as needed.