Don’t use onomatopoeia. You know, when you’re reading a book, and you come across a part where a gunshot suddenly rings out, so you see written in the book: “BOOM! KABAM!” Or something loud falls, and you see it described by the word, “KA-POW!” Or the character can hear a calming stream: “swisssssh, swissssssh.” Perhaps as a writer, you’ve worked some onomatopoeia in to give readers a sense of the sound you want to convey.
It’s like reading a Batman TV show. Don’t do it. It’s bad.
It’s bad because it’s awkward, and it takes you out of the reading experience, and it doesn’t really sound like the sound you want a reader to hear. There’s an instinct to describe sound as much as any of the senses, of course, but that’s not what onomatopoeia is doing.
One of the underlying conceits of written narrative is there’s a voice, a speaker or narrator, even though good writing makes you forget that when you get really involved in the action. When you come across the written out “boom” or “ka-swish,” it’s not the sound you’re seeing- it’s the narrator’s voice going “boom” or “ka-swish,” like you’re listening in your head to somebody make sound effects with their mouth. You’re literally spelling out the sounds a human mouth would make when imitating such a noise. Think about that. It’s ridiculous.
Onomatopoeia makes the whole scene silly. Try to describe the sound, maybe with a good simile (“The shot rang out like concrete crashing to the ground”), or just say a sound happened, clear and simple (“The river trickled gently, burbling around the bend”). Or just say the action and assume your readers know sounds accompany actions – why not? Most people can imagine what a gunshot sounds like, if you say, “The gun fired.”
There’s no need to write out in letters how the sound-effects-mouth guy from Police Academy would transcribe it in court.