Open a new file in Garageband 

Select the yellow Drummer track

Choose a drummer style and sound of your choice on the left

Ensure it lasts for 8 bars (upto bar 9)

Add a new track using the + at the top

Choose a suitable sound for chords (like a Synth Pad)

Choose a Chord Progression from below

Create a MIDI region (right click)

Programme in your notes as shown in the video

Choose a contrasting Chord Progression from the ones above (or create your own)

Program this in for bar 9 to 13 for your Section B

Copy your Section A chords to the end to give you your Ternary Form

Extend your drum beat to cover all the chords

Create a new track using the '+' and select an appropriate bass sound

Use the name of each chord as the Bass note (ie. G major has a G bass note) and record a bass pattern

Adjust the volume of each track and balance it up