I'll usually find something noodling around either in my head or while practicing some licks or some chord riff will sound cool I will then take it and put it on a track that will be a scratch track to help find the other parts of the song through the other instruments I have a friend that likes to use a metronome (the one built into pt so that he can keep everything in time from the very beginning this is a good idea?... also have an old BR 1600 CD recorder that has built-in drums. I very easily and quickly can draw up a drumbeat and change the tempo however I want and then just let my fingers do the riffing. The one thing I usually have trouble with his finding a good Bridge the other parts usually come pretty easily for me but it is the bridge that I always have trouble coming up with.