Choose a sentence in which to add a metaphor. The sentence should be a descriptive sentence, not a factual statement.
Think of what the action or object in the sentence reminds you. For example, if the sentence is "My brother was very angry," think about how he looked or acted. Picture animals, elements of nature, even machinery that he reminded you of. Don't just think of an object, try to think of a descriptive word to go with it. For example, a roaring tiger versus just a tiger, or a raging wildfire instead of a fire.
Incorporate the image you thought of into the sentence. For example, "My brother was a roaring tiger." Make sure you write the sentence as if the object of the metaphor actually is the thing you are comparing it to.