Or 6, if your character is a psychic. Or 7. Or whatever the number. As long as you mention at least a couple!
What's about to follow is a rant brought on by all this editing I'm doing right now, and by reading writing samples some friends send me, and by reading writing submissions to @ellowrites. Sure, there are other, BIGGER, more important parts to every story, like the characters and the plot and the dialogue, but without a sense of atmosphere you're fucked. Some people call it setting. Some call it world building. Doesn't matter what you call it. It has to be there, at all times. It's the place where your story happens.
1. What do we see?
This is usually the easiest thing to describe and rarely gets forgotten. Is it dark or light? High or low? Pretty or ugly? What size are things? How far or close are they? What shape? Color? How many? Think of describing a movie. Your story is a movie in your head. What do you want us readers to see?
2. What do we hear?
Often sound is completely left out, and that's a shame. With sound you can create large or small spaces. A sense of horror. A sense of warmth and bliss. How does the speech of your characters differ? High-pitched? Raspy? Is there wind? A ticking of a clock? A beating of a heart? Nervous foot tapping? Cars honking in the street? Children playing and catcalling? Maybe music? Birds? Echo? Softly falling snow? It's amazing how much you can do with sound alone.
3. What do we smell?
Smells evoke powerful memories. If you don't mention smells in your book, you're missing out on an opportunity to make a lasting impression on your reader. Is the character's breath stinky? Sour? Does the kitchen smell like cookies or stale soup? Is there a whiff of something rotting or blooming? Or cooking? Or is there a smell of freshly turned earth? Of rain? Glue? Fire? Gasoline? Blood? Animals? Vomit? The smells can be endless.
4. What do we taste?
Sometimes the smells are so intense, you can taste them. Use it. Describe the taste in someone's mouth as bitter if they're afraid (or sour or something). Describe things as having tastes, like a building white as sugar, or as salt, or the air acrid with smoke that gets in your throat. Food and drink, of course, go here. But you can twist it any way you want and go nuts.
5. What do we feel?
Is it cold? Hot? Humid? Are the clothes scratchy? Is the seat hard or soft or does it have sharp bumps? I'm talking about the skin here, not some sixth sense (we'll come to that). Is the touch light and soft or is it a swift, sharp blow? There is so much you can tell here, with simple little details that will add credence to your story.
6. What do we sense?
Okay, okay. See? I told you we'd get to this. So, if your characters have special powers, please don't neglect to have them sense extra stuff that normal humans don't. This could also apply to characters that are animals or aliens or super heroes or witches and wizards, or you could even apply it to children and elders, this special sense that adults tend to lose—intuition, premonition, foreboding, sensing stuff with the backs of their heads and more in that same realm. If you're building a fantasy world in your book, a can do a lot with very little, simply by having your characters sense stuff and giving it off as normal. That's when we will start believing in your fantasy. That's when we'll start feeling it's real.
Okay, now to the "less-is-more" part.
YOU DON'T HAVE TO MENTION ALL 5 (OR 6) SENSES AT ONCE.
Pick one or two or three, and that's enough. Just make sure that in the course of your book you touch on all of them as needed. For example, if you're writing a fight scene where a hero is fighting a beast, think what could convey the atmosphere of that scene the best. What's the strongest sense in a fight? Eyesight, yes. Hearing, yes. But also a sharp sense of smell. The beast probably stinks. The hero probably sweats. With one quick sentence you can make it way more real than forever describing the terrible toothy maw or the twisted horrified face. Take another scene, someone dying. You can do so much with sound here. The last breath, how long it was or how short, was it wheezing or gasping or hitching? You get the drift. (Good romance writers know how to use taste, read a couple romance books to see how they do it.)
No, wait, I lied. I'll tell you one more thing. Before I got this thing about senses pounded into my brain, I had a little cheat sheet hanging on the wall in front of my laptop. It listed all 5 senses, so when I would glance away from writing and look up, I'd remember. Steal this idea. See if that helps you. If it does, you owe me cookies.