Sketch the bare facts of a scene. For instance, a bland, no-nonsense example could be someone staring out a window on a rainy day.
Choose a mood that you want to create. Rainy days lend themselves to horror and mystery, but you can also create a mood of hope or inner reflection, if you want to get creative with it.
Use the idea of the mood to choose proper lines and words to show the reader the scene through the lens of your desired mood. For example, a dark and noir mood might sound like, "The sky opened up and cried like all the tears of all the children without mothers wept at once, hiding the world from the heavens. It was a good day to do bad things." Alternatively, if you wanted to create a feeling of hope, the line might read "The rain came down like a small flood, washing the dirt and the heat and the muck from the world. It pulled back the filth and showed the beauty that we all knew was lurking beneath even if it was rarely seen."
Use dialogue to bring out a character's personality and thoughts, and to add those to the scene. How a person speaks, the accent they use, the emotions present and what the choice of words is can all add to the mood of the story. A line like "'You cannot kill what is already dead. You can only trap it, and pray that it doesn't get out and take off your hand before you turn the key' the holy man whispered, his eyes darting about as if the restless ghost might head him and come." In this case the choice of whispering reflects an air of tension, and the character's proper speech represents that he is from a certain class and education level. All of this can help add to the mood of a scene.