Well, I usually start with a general idea of the feel/type of music and start with a drum beat or bassline, then add more channels/instruments as I go along, even experimenting with different sounds.
Sometimes, I think of a tune (usually away from home) and scribble it down in LSDJ to pick it up later in Famitracker. Not everything turns out as expected, though.
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Everything moves real slow when it's 40 below.
I hum to myself in the shower and if I hear good sounds I keep humming so I don't forget, then I run to the computer and open up Famitracker and transcribe it. I call this process "rinse and repeat."