Via Superknob, I'd like to add Part 2 Organ + Part 3 Strings, weaving in and out of the song.  At times Part 1 + 2, and other times Parts 1 + 2 +3.  I'd like the volume of Parts 1 and 2 increase gradually.  Where can I find a step-by-step process to do this?  So far, the YouTube videos are not helping.  Thank you.