A little practice. The textured spheres and a color board/ mood study. They were both fun to do, though some textures, like leather are difficult to transfer to a sphere like object. You recognize the structure of leather way easier when you can see some folds.
Besides the textures, painting spheres is a nice practice too. You have to think a lot about shadow, reflected light and edges to make a sphere look as if it has real volume. To my surprise it is not as easy as I thought. ^^