1. Create a DUPLICATE track of the piano. In GarageBand, you'll get an empty track that contains the exact sound (piano type, reverb, etc), of the original. Other programs copy the piano notes as well; for those, simply delete all music.
2. Prepare this new INTRO track: Record a couple of notes -- any notes. In the staff music window, extend the length of the recorded track out about 12 measures. Then delete the recorded notes. They were just placeholders.
***** ENSURE THAT "SNAP TO GRID" IS ON *****
3. Go to the main piano track and DECIDE ON your favorite PASSAGE to begin your song. It's easiest to select an odd number of measures, leaving a final even measure to hold a note before "the singing begins."
4. COPY those notes-- including the pedal marks that look like fancy L's at the bottom. Then put the playhead line exactly on Count 1 where the intro will begin, and PASTE the notes into the INTRO track. HINT: Sometimes it's nice to leave a couple of measures at the beginning for the drums or flute to bring the song in before the piano starts.
5. SELECT ALL NOTES BUT *NOT* the pedal marks. Then put the cursor on a note that's on count 1, hold down the note firmly, and slide up that note exactly one octave; all other notes will follow if they have stayed selected (green highlight in GarageBand).
ANSWER this QUESTION: Do you want to merge the piano tracks? Completely depends on your preference for a fast way to print the STAFF (versus creating a separate track called SCORE) or to have a "clean" looking piano track.
ADVANTAGE of separate tracks:
a. Fast flexibility in sound... for example, you may
want a more "ethereal" sound with a slight echo.
b. ALSO -- during the initial recording, both the producer and vocalist have a visual clue of when to begin singing:)