Brainstorm. Sometimes, metaphors spring spontaneously to mind, However, more often metaphors arise from the effort of attempting to explain an experience, a process or a concept to ourselves or to another. Geary compares the mechanics of metaphor to the mathematical equation "x = y" -- where hope is a thing with feathers rather than being like it.
Develop your metaphor. Use maps, doodles, free-writing or try writing a letter to someone you feel comfortable with to elaborate upon your metaphor. See how the metaphor comes to life using nouns, verbs and personification. If you like, you can use the worksheet at the website Drawing Metaphors or create your own.
Write a first draft using your ideas, your notes and your language to make clear and understand the first concept in the context of another, your metaphor. The metaphor should be sustained throughout the written text, using verbs, personification and other devices that work, as in Carl Sandburg's poem, "The Fog": "The fog comes / on little cat feet. / It sits looking / over harbor and city / on silent haunches / and then moves on."
Rewrite. Rewrite to enhance the metaphor structurally and at the sentence level. A good extended metaphor should illuminate the concept it explains rather than drawing attention to itself or confusing the reader. It should evoke the right mood. A powerful metaphor encourages us to think outside the box, to illuminate possibilities and discover new patterns. Eliminate what is clumsy or inorganic to shape the idea to make it shine.