Writing is an artform that improves with practice, but sometimes the words just won't come out of your mind at the moment you desire. You wonder how on Earth particular authors wrote such large novels with exquisitely chosen words to be able to make readers feel like they're directly in the story themselves. After trial and error, I've discovered what works best for my writing.
Disengage from the world. That's right. You have to put all of your household chores, errands, problems, social media, and everything else vying for your attention on the back burner for just a little while. Don't worry, you'll come back to them and be a responsible adult later. Just carve yourself out two or three hours of just you time. Meet with the keyboard as if it's a date. Give it your full attention. You won't be sorry.
Put on some music. Not relaxing music or you'll just want to take a nap. Pick something that makes you feel what your characters are feeling. Put the earbuds in your ears and pretend the rest of the world isn't around you. Feel the angst. Feel the heartache. Whatever the singer is pouring their heart out about - the words of their own creation, their own story. Feel it.
Sometimes you need to visually see the characters or the setting before you're able to write a detailed account of either. That's when I scroll through Instagram or Pinterest. If I need a character's demeanor, mannerisms, voice, etc. I watch, or rewatch, a TV show or movie. Sometimes you need to connect to a real person to be able to create a person that's real to you. One that will be just as real to the readers.
Snacks. But not too many. (Calories and cavities aren't cool.) And something relaxing like coffee or tea...or wine if you're writing a spicy scene.
Let yourself be open to daydreams. Let the scene play out in your mind and feel what the characters are feeling during it. If it's more than one scene, write them down in a notebook (you know you collect those things like a packrat - this is your reason to use them). This is where character dialogue flows effortlessly. Your characters are cast by you, and you can make them do and say anything that makes the story progress from scene to scene. Just daydream to your heart's content. The story will unfold effortlessly.