To write a metaphor, compare two very different things as if they share a common characteristic, but without using the words "like" or "as." Including "like" or "as" in a comparison makes a simile. "The lake was a shining sheet of glass" is a metaphor; a simile would state that "the lake was like a shining sheet of glass."
Direct metaphors mention both subjects that are being compared, while implied metaphors indicate only one subject. "Bob barked orders to his employees" compares Bob to a dog without saying so directly, so it is an implied metaphor. Conversely, implicit metaphors state the two things being compared, but don't specify their similar characteristic. The sentence, "The house was a freezer after the furnace went out during the snowstorm," requires the reader to draw the conclusion that the house is very cold. An explicit metaphor might say that "the ice-cold house was a freezer," making it clear that both the house and the freezer are cold.
When writing metaphors, make them clear enough that the reader can relate to and understand the comparisons. However, metaphors should also be unique and original. Both similes and metaphors are types of figurative language that help readers gain insight about the writer's perspective. Numerous adjectives can describe sunshine, but the sentence "The sun's rays were spears of light" conveys a more nuanced understanding that the light is blinding or even dangerous.