I find the easiest way is to start with a drum loop and just improvise something on synth or guitar synth. Then start adding overdubs or just use the midi you already recorded to add more synth parts using different instruments.
Then add the guitar and bass parts and start cutting and pasting parts around until you get something that works.
Last step is to grab the thesaurus and rhyming dictionaries and come up with some lyrics.
Hell, I did one song just using playing an A chord on the guitar synth and running it through about 2 dozen patches in SampleTank.

http://s93744050.onlinehome.us/SchmangeOnline/DarkBattle.mp3

It sounds like an orchestra but it only took about 15 minutes.