Start a loop in session view of the length you want, load that with an empty midi clip with the synth/sound you want. Then turn on the monitoring button on the left of the midi piano roll so that when you click the notes on the left of the piano roll you can hear them. When you find the right note, draw it in in the right place. Gradually build up the loop with the right notes and timing while you're running it in a loop. Or use you keyboard with record off, find he notes, and then draw those notes that fit in with me mouse in the right place?
Try the same for drums. Then make more clips using the same and other sounds. Jam about with them in session view til it's taking form.
Voila! After hours of experimenting, learning, jamming about, improving, thinking, listening, tweaking and re-doing: a song!
