When you have everything mixed down together, go to single track mode. Highlight where you want the fading out to start, to where you want it to end (doesn't have to be the end of the instrumental, you can cut away what you don't want )Once it's highlighted, go up to favorites and you will see fade out. Listen to it to make sure you got it how you want it before you do anything else cause you can always go to Edit and undo it...

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