I cannot stress how important this is. These two will be covered in the same chapter because they go hand in hand. Let me walk you through it.

When you write a description, it may be of an action, or a setting, or an object, or an emotion. No matter which one you are writing, descriptions are always harder than dialogue and must be treated with care.

Now, when you write a description, you will simultaneously be creating an atmosphere. Here is a descriptive piece I wrote for school as an example.

The memory is clear blue and dry red and dusty green, swirling slowly in the vial, dancing and capering until my eyes swim. The glass emits a faint warmth, dry and fuzzy, and it feels like sunlight. I touch the cork it feels like jagged rock, it feels like sharp-edged grass, it feels like dry sand and I have to pull my finger away. I hold it up and sounds flow through my mind. The memory sounds like the wind, like the crunch of footsteps, like distant cries of animals. Tentatively, I twist the cork, and it comes out as easily as if it had been waiting for me.

The dirt trail is dusty beneath my feet. A few steps ahead, it vanishes entirely, leaving me to wade through the long grass. I feel the coarseness against my shoes, feel the sand rubbing against my soles. I look up, and the sun greets me, bathing me in muzzy warmth and I blink sleep out of my eyes. The sky is temptingly clear, a colour that is turquoise at one horizon and fades to cyan at the other. The ground slopes up, and then I am on a hill, and the dry, faded green stretches out forever in front of me. I reach out, and my fingertips caress the breeze. A herd of guanacos shy away from me, letting out shrill shrieks that once made me laugh. Now I just smile and watch them canter away.

Out of the corner of my eye, striking orange catches my eye and I turn. The slope looms, the grass abruptly falling away as if it has failed the climb. I approach it and press my hand to the ground. It's warm, the rusty red dust clinging to my palm. I climb, and soon reach the bloodred rock where I sat, sipping soup from a thermos flask. I ghost my fingers over it, brushing away more copper-hued grit.

The wind murmurs in my ear and I lift my head. Below me, the grass stretches on, from horizon to horizon with dotted glimpses of golden-grey sand and brown-red earth amongst the rustling stems. The sun gives way to biting chilly wind, the wind fades to allow the beaming sun.

I close my eyes and fade.

The memory is clear blue and dry red and dusty green, swirling slowly in the vial, dancing and capering until my eyes swim.

This piece was based on a prompt: "If you could bottle up a memory and open the bottle to relive it, what memory would you use?"
Reading this piece, you can tell that I have purposely created a very dreamlike atmosphere. I emphasised the sleepy effect of sunlight, focused on colour, until the scene seems both vivid and distant at the same time. Does this look hard? It's not! I have not used many complex words or phrases at all. The simplicity of the text also contributes to the atmosphere.
How to create such an atmosphere is quite hard to describe. I will say this—when trying to create a certain atmosphere, shut out the world around you. Use your imagination, create that atmosphere in your mind. And the words will work. Don't try to write dreamlike scenes while agitated.

Hopefully, this example and my explanation will have helped you. See you next time!