Depends on what I'm starting from.
If I'm starting with drums or a bassline, I'll start building up a 2-4 bar loop, duplicate it and make some changes, then repeat until I have a nice 16 bar loop that you could listen to for like an hour without really getting bored. Then I'll build the other elements around it so that they accent the focal point.
If I'm starting with chords or a melody, I'll start with a piano VST and just noodle around with chord progressions until I have a 4, 8, or 16 bar idea, then I build the melody on top of it (and just double the root down to the bass of the piano). I get a simple drum loop going, and arrange my track like normal. Then I'll split up the various notes to be different instruments and start looking for a sound that fits the lead line or the chords, and when something fits the idea I had for the song I'll just start refining from there.