Chord Trainer LessonsIn addition to basic guitar lessons, GarageBand has a built-in Chord Trainer that helps you practice guitar chords.
The trainer is designed to be like a game that tests your ability to play the correct chord before you can progress to the next one.
Access the Chord Trainer
In the GarageBand start window, select Learn to Play and then choose the Chord Trainer option.
Play the Chords
The Chord Trainer will take you to a similar screen as the guitar lessons, but without a teacher. Instead, you will see a diagram of the chord to play and how to play it. The bottom timeline has the progression of chords that you must play.
When your guitar is plugged in, GarageBand will read the chord you play and signal whether you have played it correctly or not.
If you have played it 
correctly, the chord diagram will light up green and allow you to progress to the next chord. If you have played incorrectly, you will need to keep trying until you do it correctly.
correctly, the chord diagram will light up green and allow you to progress to the next chord. If you have played incorrectly, you will need to keep trying until you do it correctly. GarageBand also uses a timer in the bottom left corner to keep track of how long it takes you to progress through the chords.