Interesting posts thus far, it's amazing how everyone has their own approach!

It really depends on what I am writing. Is it a commissioned piece for someone else? Is there a deadline? Is it background music for a soundtrack? Is it one of my original projects, and if it is, is it meant to be tracked live off the floor or is it being built up track by track or via file sharing with studio musicians?

Once that's out of the way, I usually think about what I am trying to say with a particular piece and then play the notes, scales, chords, that best reflect that emotion. I usually start with the bass guitar or the bass part because the bass is the most important part; it steers the ship. Sometimes I think rhythmically; such as a time signature that sounds interesting or a polyrhythm I like.

From there it's a pretty quick process; I tend to write quick and I don't belabor the details, I just let it say what it wants to say and then move on.