Simile: a comparison of two unlike things using "like" or "as."
Metaphor: A direct comparison of two unlike things. Essentially, a simile that doesn't use "like" or "as."
Now that we've defined those, why do we use them? Why can't we just say the sky was cloudy? Or the music was calm?
First, those are super boring. Second, they don't give the reader a picture. People like images and "cloudy" isn't a good image. But saying the sky looked like a glass of spoiled milk? The reader can see that.
Using a comparison also gives you a chance to associate emotion to what you're describing. If I say the sky looked like a glass of spoiled milk, you are not going to assume the sky was "pretty" or "happy" that day. You're probably thinking along the lines of "gross." If I'd said it was cloudy, you would have had a different feeling.
Now, how do you use similes and metaphors? First rule is not to overuse. Not every adjective needs a simile. Sometimes it's enough to say her voice was loud or the walls were pastel blue. But if specificity is needed, a comparison is appropriate. Let's go back to the example above. There are lots of types of cloudy. Without a simile, the reader won't be picturing the same thing as the writer. But if I say the room was pastel blue, they will be. There's not enough interpretation around pastel blue to justify a comparison.
Second rule is to be unique. It makes me sad when people use cliche or common comparisons in their writing because similes and metaphors are excellent opportunities for characterization. Especially if you're writing in first person. What would that character see? What would they associate with what you're trying to describe? Throw out the first one you think of. Probably the second one too. Maybe the third.
Third, don't compare it to something similar. Remember the definition? Comparing unlike things. Stop thinking about what the thing you're describing with the five senses (look, taste, touch, smell, and sound). Think about the emotions associated with it and pick something else with the same emotions.
Like yellow. Lots of things are yellow. Bananas, fire hydrants, traffic lights, sunflowers, etc. All of those have different emotions. If I'm trying to describe a room that's bright yellow and it's a happy yellow, I might compare it to a smiley face sticker. If it's sad, I might compare it to a sun-stained wall. If it's overwhelming, I might say its like looking directly at the sun. See the difference?
Let me give you some examples from my own writing.
In Chaos in G Major, I describe piano music several times. I can't make the reader hear exactly what I'm hearing, but I can use similes and metaphors to make them feel what I'm feeling.
Olivia dad plays a piece for her and to describe it, Olivia says "the music swelled and fell like a deep, sleepy breath."
I could have said the music was calm, but then the reader isn't hearing what I am. Calm can sound like a lot of things and as soon as readers see the words, they'll make their own connections. So I thought about the emotion. I knew this was a lullaby for an infant. I associated it with the quiet of the moment before a child falls asleep. A deep, sleepy breath.
At that point, the reader is free to make their own connections. A deep, sleepy breath played on a piano can sound however they want it to. It doesn't matter if they hear what I hear. They're feeling what I feel. And that, my friends, is the point of similes and metaphors.
Exercise 1:
Do a search in your writing for metaphors and similes. Think of the emotion you're trying to associate and change the metaphors as needed.
Exercise 2:
Pick something from your writing to write a simile or metaphor for. Write ten and make the meaning of the object of the comparison change with each. Can you make what you're describing happy, sad, angry, bittersweet, and stressful? What else?
Exercise 3:
Make a list of your POV character's favorite things. How can you work those into a metaphor that also characterizes them?