Normally I'll start by laying down a few measures of a rhythm and playing around with instrumentation. At this point I'm not worried about whether any of what I'm playing will make it into the actual song, I'm just looking to see what sounds good together and get an idea for what kind of tone/genre I'm going for. Then I'll just jam out for a bit with various instruments until I find something I want to use for a melody. Sometimes I'll use these test measures in the song, sometimes not, but I find it easier to develop musical ideas by gathering together the instruments I wish to use first. Plus, I can save time and maintain tonal consistency through the album by saving the instrumentation as a preset.