Choose an object (or an animal). Research it. Study it so you understand its defining features.
Then, draw it. Paint it, collage it, render it in Photoshop. Try it quickly, try it without looking, try poring over its details.
Draft it until your head spins, then draft it some more.
Refine your drafts, choose the best of them, and finally, present them on an 11″ x 17″ poster.