Learning to read music, playing with an instrument, and writing down what sounds good are the fastest ways I know.
If you don't have access to an actual instrument, the program Bosca Ceoil is fairly easy to use (especially for pieces with a lot of repetition), completely free, and translates your notes directly to sounds even while continuing to loop through the portion of music you're fine-tuning.
I also just downloaded MuseScore, and it's looking more user-friendly than I feared, but still much slower (without a MIDI keyboard), feels more limited in instrumentation, and requires more precision and music notation knowledge than Bosca Ceoil.