I usually start with a sketch or challenge of some sort. Then pick the stamps, then a patterned paper or two or three, then grab solids to match and start thinking about embellishments. I work with a large sheet of grid paper on top of a cutting mat on my desk, so then I just start sketching the card out on the paper, and note what papers I want to use where. It helps me "see" it in my head all finished if I know what colors will be where.
Designing for Reverse Confetti, The Cat's Pajamas, and Verve