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What characteristics does it have? What does it do? How does it make you feel? Does it have a smell or taste? Brainstorm by writing down whatever descriptions come to mind. Don't get bogged down by obvious details; metaphor is all about thinking outside the box. For example, if you want to write a metaphor about "time," try writing down as many characteristics as possible: slow, fast, dark, space, relativity, heavy, elastic, progress, change, man-made, evolution, time-out, timer, race, run. Don't self-edit too heavily in this step; your goal is to generate a bunch of information for yourself to use. You can always scrap ideas that don't work later. Jot down lots of other things that share some of these qualities, but again, don't be too linear; the less obvious the association, the more interesting the metaphor. If you're writing a metaphor about a concept, flex your brain by trying to equate it with an object. For example, if your topic is justice, ask yourself what kind of animal it would be. Avoid cliches. As Salvador Dali said, "The first man to compare the cheeks of a young woman to a rose was obviously a poet; the first to repeat it was possibly an idiot." The goal of metaphors should be to convey your meaning with impact and originality in a compact package: the single intense bite of sea-salted caramel chocolate gelato vs. a whole bowl of bland vanilla froyo. This is a brainstorming activity, so let your imagination run wild! For the "time" example, free-associations could be ideas like: rubber band, space, 2001, abyss, enemy, ticking clock, weight, wait, loss, adaptation, changes, stretching, returning. Is there a particular tone you'd like to set or maintain? Does your metaphor need to suit the larger context of whatever it is you're writing? Use this to weed associations out of your list. For the "time" example, let's go with "celestial/spiritual" for the mood. Eliminate ideas that don't fit with that mood as you develop your ideas: for the "time" example, you might scratch out enemy, 2001, weight, and ticking clock, as these are all fairly "earthly" ideas. Try to keep the nuances of your chosen topic in mind. For example, if you're comparing the concept of justice to an animal, a "prowling leopard" conveys a very different idea of what you mean by "justice" than an image like a "weary elephant." Both of these are probably still more apt than using a "newborn kitten," though. Write a few sentences, a paragraph, or a page comparing your original topic to some of the associations you came up with. Don't worry about forming metaphors just yet; focus on the ideas and see where they take you. For the "time" example, this step could generate a sentence like the following: "Time is the rubber band, shooting me out into the unknown then bringing me back to center." This sentence has taken one of the ideas from Step 2 and has started attributing concrete actions and characteristics to it -- the starting-place of a metaphor. Since metaphor draws attention to the mechanics of the language, it's important that your phrasing literally sound right. A metaphor conveying softness shouldn't have a lot of harsh consonants; one describing depth might include deeper vowel tones (ohh and umm); one conveying redundancy might include alliteration (i.e. repeated sounds); etc. In the example sentence generated in Step 4, the basic idea is there, but the words don't have much power behind them. For example, there's very little alliteration, which might be useful to employ if you want to convey a sense of repetition. The idea of the "rubber band" also suggests something or someone firing the rubber band, which detracts from the metaphor's focus on Time performing the action. Write a metaphorical sentence equating your original topic with one of your other objects or concepts. Does it make sense? Is it original? Does the sound match the feeling? Will a different one sound better? Don't accept the first thing that works; be willing to discard an idea if a better one comes to mind. For example, adding in alliteration and providing an action for Time that is more independent could result in a sentence like this: "Time is an endless rollercoaster ride; it stops for no one." Now, the focus is entirely on time, and the alliteration of the repeated r sound adds to the sense of repetition that the metaphor's getting at. Metaphors are often used as nouns -- "her face was a picture," "every word was a fist" -- but they can also be employed as other parts of speech, often with surprising and powerful effects. Using metaphors as verbs can give actions more punch (sometimes literally!): "The news clutched her throat in its iron fist" expresses a more intense feeling than "She felt like she couldn't breathe." Using metaphors as adjectives and adverbs can vividly characterize objects, people, and concepts in just a few words: "The teacher's carnivorous pen devoured the student essays and belched up the occasional bloodstained comment" conveys the idea that the teacher's pen (itself a metonym for the teacher) is tearing these essays apart and eating them, leaving only a mess of blood and guts once it's finished. Using metaphors as prepositional phrases can describe the feel of actions as well the thoughts behind them: "Emily examined her sister's outfit with a surgeon's eye" suggests that Emily believes she's a trained expert in fashion, that she has a meticulous eye for detail, and that she sees her sister's outfit as a potential disease to be cut off if necessary (perhaps not something that makes her sister happy). Using metaphors as appositives (nouns or noun phrases that rename a nearby noun) or modifiers can add literary polish and creativity to your work: "Homer Simpson sidled onward, a yellow-domed pear wearing pants."
Think imaginatively about what you're trying to describe. Free-associate. Decide what kind of mood you'd like to set. Run with it. Read everything aloud. Transform your comparisons into metaphors. Stretch your ideas.