The procedure for making a MIDI hymn is (simply) as follows:
- Set the key of Garageband to match the key of your hymn (see red annotations in second screen shot). Input the four voices of the hymn Glory Be to Jesus (soprano, alto, tenor, bass) into separate tracks in Garageband.
First four tracks in Garageband - Tracks must be green (software instruments) to enter MIDI information in Garageband. Notes can either be clicked into regions by first command-clicking (hold down the Command key while clicking with your mouse) in an empty track, then command-clicking onto the staff in score view as shown below:
Adding MIDI notes to a track - After entering the SATB tracks, then it's time to start researching the ranges of orchestral instruments and their families (woodwind, brass, and string) to see which instruments can play in the ranges of the soprano, alto, tenor, and bass voices. A good source for finding the ranges of orchestral instruments is Wikipedia.
Wikipedia: Trumpet range - We've been using Apple's Jam Pack 4: Symphony Orchestra (manual) to get high quality samples of real orchestral instruments in Garageband '11 (6.0.5) in our Music Tech lab. When you double-click on a track header (or click the "i" button in the lower right corner of your screen) you'll be presented with the instrument panel. Choose "Jam Pack Symphony Orchestra" from the dropdown menu, select the instrument family you need (Horns = Brass) and select the orchestral instrument that can has the same range as that voice of the hymn.
Selecting Orchestral Instruments in Garageband '11(6.0.5)