Creating atmosphere when writing is easy.

Adjectives
That speak to the senses, colour, texture, sound, smell, space.

The rancid stench of death enveloped me in its rank and rotting embrace.

Adverbs
That describe emotion-  caused by action, nervously, anxiously, worryingly, confidently.

He looked around nervously, his mind whirling out of control into a neverending spiral of despair and raw fear.

Non defining clauses
Add extra atmosphere with short non defining clauses.

He held her by the throat, his fingers digging deep into the sinews of her neck, and he knew at that moment she had to die. There was no way back from this place, from this moment…

Pace and timing
e.g. suddenly, instantly, meanwhile, throughout, just then, later, moments later etc.

She pushed herself from the ledge without hesitation, her bare feet suddenly leaving reassuring rock and into the uncertainty of air.

Sentence fragments
Sentence fragments, or short sentences sometimes say more than longer ones, especially when used after complex sentences that focus on detail. They instantly change the pace.

She pushed herself from the ledge without hesitation, her bare feet suddenly leaving reassuring rock and into the uncertainty of air. Nothingness. Emptiness. Breathlessness as freefall reached into her lungs and drew it from her like…