First, read Andrea Cohen’s Circular, from Copper Nickel.
Then write a poem that mentions or uses a shape in some way (a circle, or a square, or a triangle, or if you’re daring, a dodecahedron….) The word could refer to a love triangle, or perhaps someone is being uncool, a real square. You could alternatively draw the shape on a blank page and try to write to fill that shape. How does the shape influence your choice of words?