You can use a simple random draw game to practice your use of metaphors by writing two lists of words, with both lists using either prepositional phrases, verbs or adjectives. Cut the lists into slips of paper, and place them in two separate hats. Draw one from each, and practice writing about one as a metaphor for the other in a short writing assignment. You can reverse the two words for another challenge or draw two more words and start again. This helps you practice forming simple metaphors.
Poetry relies on careful metaphors to deliver a setting, theme and idea in a unique way that a reader can understand in a familiar way. You can construct your own poetry by developing an idea through your own careful metaphors. Explain a complicated idea to your reader using a metaphor for something that they can easily understand. You can combine your frustrations with writing metaphors with their frustrations about reading poetry or your fatigue with being a student with their fatigue from issues in their lives.
You can use a complex story metaphor to retell an older story. Select an older short story, something you enjoyed reading or found particularly interesting. Modernize the story by updating the personalities in the original with post-modern ideas, rethinking the careers of the people from the original and re-creating their lives in the modern world. Allow the story to change as the world in which it occurs changes and retell a modern version of your old favorite. Your characters become metaphors for the original characters as your entire story becomes an elaborate metaphor for its original.
Complex metaphors are possible, but they require practice and expertise. You can practice this process by beginning with a long, but simple, sentence. Select an emotion or theme to begin. Rewrite the sentence four different times, each time creating a different metaphor within the original sentence that corresponds to your emotion or theme. The first should be a metaphor for the verb in your sentence, then the adjective, then the prepositional phrase and finally for the modifier. Focus on how each sentence seems different, even though they are all working toward a similar theme.