my band, sometimes its a riff someone brings to practice. occassionally somone will write all the parts, but usually that ends up with people getting mad 'cause they don't like their part. hopefully we have the old reel-to-reel recorder running to pick up stuff that we play so we dont forget it. Most likely one of us will bring one riff, then we will make the rest of the riffs for the song on the spot, using the original riff as the style guide. I don't even try to do drums, the drummer can figure that out on his own.