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Last time we tracked an electric guitar in Garageband, using the on-board amps and effects. Today we'll put down some acoustic guitar, with as natural a sound as we can get. I'm going to plug in a Gibson J-45 with a Fishman Natural Matrix pickup in it.
The first step is to click on "Track" in the toolbar and select "New Track"...or simply press "Command", "Option" ,"N" as a shortcut. This gives us the prompt to choose from three different kinds of tracks - "Software Instrument", "Real Instrument" or "Electric Guitar" (Fig. 1). We'll chose 'Real Instrument", even though we are plugging in because we want the straight sound of the guitar without the amps and effects options of the Electric Guitar track. If we were using a microphone to record the acoustic, we would also use the "Real Instrument" track.
(Figure 1)

This creates a generic blank track that is titled "No Effects" by default (Fig. 2). We can slide over to the info pane (if it is not open, "click the "i" button at the bottom right of your screen).
(Figure 2)

The first choice on the drop-down list is Acoustic Guitar, with five presets for getting the sound you want (Fig. 3). We want it dry and clean, so we'll choose "Natural"
(Figure 3)

This gives us a window (Figure 4) to select which input on our interface we will use, and whether to monitor the input or not. I'm using input Mono 1 of my ProFire 610.
(Figure 4)

Click the Region Cycle button to the right of the counter (Fig. 5) and select 4 bars on the timeline (Fig. 6). Once we hit record, Garageband will cycle around the four bars until we have enough good takes to choose from.
(Figure 5)

(Figure 6)

Here I've decided that Take 4 is the best, so we'll use that on our song (Fig. 7).
(Figure 7)

That's it for now. Next time we record a vocal...!