1 - Outline
2 - Volumes: define what's inside, outside, above and below using colours that are far from each other in the palette
3 - Wood Texture: this is the hardest part, the key is to understand what makes wood a good wood, so you are have to try to render wood veins, wood nods and so on
4 - Out-of-palette details: it's important to add little and not too visible details using colours that are similar to the sprite palette. I do this step because each real texture has more colours than a single colour scale