Hmm, I mean I would start with a concept, just sketch up some ideas, sketch some mood colors as well and then just start paintin, the rendering is just rendering, I feel like you have to do some concept art and figure out what you want to do with the mushrooms, like what kind of world do they live in? Study:
Spent an hour playing around in photoshop, todays focus was on colour and presentation rather than rendering out the drawings. As always critique is much appreciated