Choose a subject that has an iconic silhouette shape. For example, a house looks like a square with a triangle on top. A sun is a filled in circle with lines extending from it. A t-shirt is shaped like a wide capital T.
Write about the subject you have chosen. Enjambment is not important at this stage. The more poetry you generate here, the easier the next step will be.
If you are writing on the computer, use your word processor's formatting tools to arrange the text into the silhouette shape of your object. The more simple the shape, the easier this will be. For more complicated shapes, you may want to use a program like Photoshop. If you are using pen and paper, it may take several tries to make the poem fit into a specific shape.