Open GarageBand. When the project selection dialog appears, click on "Magic GarageBand."
Click on the "Rock" icon and click "Choose." GarageBand will load the rock preset and the Magic GarageBand interface will appear. It resembles a stage with instruments appropriate to the chosen genre ranged around it. The instrument at center stage represents your instrument input. By default, the image will be of a piano. The other instruments are controlled by GarageBand.
Choose tones for each instrument controlled by GarageBand. Click on each in turn. When you click, a menu of available tones will appear at the bottom of the screen. For example, when you click on the guitar, a list of available tones such as "honky-tonk" and "punk" will appear. Choose a tone you like by clicking on its icon.
Preview the song by clicking "Play." The background music over which you will record a unique track will play back. Listen to it a few times to get a feel for the song.
Click on the piano representing your instrument input. A menu of options appears at the bottom of the GarageBand screen. From the drop-down menu, choose your desired instrument input. For example, if you are playing a guitar connected to an audio interface, choose the audio interface from the menu.
Click "Record" when you are ready. Anything you play will be recorded over the Magic GarageBand track. You can record as many times as you like -- each take will be saved for later listening.
Open the project in regular GarageBand by clicking on the "Open in GarageBand" icon in the lower right part of the screen. GarageBand will open the song as a standard GarageBand project. Each instrument, including your recordings, will be represented by its own track. From here, you can mix and master the track as desired.
Open the "File" menu and choose "Save" to save the track.