Open your acoustic guitar track and highlight the region in the Edit window.
Below the Track View on the track there will be a drop-down menu assigned to the Elastic Audio function.
Scroll down and select Polyphonic
You will now see the region "blink" for a moment while it adjusts to the selection. Wait for the region to return to its original visual state before continuing.
Highlight the region and select the key command (Option/Alt +0) on the keypad. This will bring up the Events window. The window opens by default to the Quantize function.
Once you have determined the rhythmic pattern of the acoustic guitar track, choose the corresponding quantization in the Quantize window. For instance, if the pattern is playing 1/8 notes, choose 1/8 notes for alignment and click Apply.
You'll now see the region conform to the grid. Now playback the region to ensure that the function worked properly. If everything checks out, change the Elastic Audio event from Polyphonic to X-Form.
X-Form is a sophisticated algorithm allowing the best possible quality from the "elasticized" audio editing.
Once selected it will bring up a plug-in window. Activate this plug-in.
Now sit back and give it time to render the file.
After it has rendered you may choose to consolidate the file. This will allow you to turn of the Elastic Audio feature from the track as it uses a significant amount of RAM.