If there's one piece of writing advice that changed everything for me, it's this: show, don't tell. Instead of telling your reader someone is sad, show them crying, slamming doors, or staring out the window.
This applies to everything. Don't say "the food was good." Describe the aroma, the flavors, the way it looked on the plate. Make your reader taste it.
Sensory Details
Great description uses all five senses: sight, sound, smell, taste, and touch. Don't just describe what things look like. What do they sound like? What do they smell like?
When you walk down a street, what do you hear? Traffic, people talking, birds singing? Capture those sounds and your writing becomes immersive.
Be Specific
Generalities are forgettable. Specifics are memorable. Instead of "a flower," write "a red rose" or "a yellow sunflower." Instead of "a building," write "a crumbling Victorian house with a sagging porch."
Specific details paint pictures in your reader's mind. That's what descriptive writing is all about.