1. Sketch it out.
2. Decide you don't like initial sketch for reasons and sketch it again fixing reasons you didn't like the first one.
3. Lay down flats and get a good idea of color scheme/lighting
4. Lasso areas for highlights and shadows
5. Start painting blending/color picking tones. I use the the brush & Gradient Fill tool set to modes normal/color dodge/linear light to paint highlights and hard light to paint shadows.
6. Add details
7. Add more details
8. Add cats.