Ever notice how the aroma of somthing stimulates a memory? Or registers a feeling within your brain? Like chocolate chip cookies baking (yum!) may remind you of the kitchen in the house you grew up in. Or freshly mowed grass may remind you of Saturday mornings as a child.
I used to wear "Sunflowers" perfume by Elizabeth Arden when I was in college. Every once in awhile I catch a whiff of it somewhere and it's like an immediate time warp back to that time in my life. I even still have a tiny bottle of it on a shelf somewhere in my bedroom with just a dash of the golden-yellow liquid left. I save it just for the memory effect.
As writers it's important we evoke all the five senses in our writing whenever we can. They are details that help create the setting and bring it to life in the reader's imagination. Sensory details should be present in all of our crafted scenes. We may be better at creating a visual or includng the sounds of a busy city. But don't forget the sense of smell! Give that busy city the odor of a full dumpster, or vendor's peanuts roasting.
We can't just use the pleasant aromas to create comfort and peace, but we need to use scents to scare or repulse our reader. Keep it sensible--meaning we shouldn't try to employ odors that no on can relate to--like a rotting alien corpse or dragon's fire. Sprinkle in scents the reader can recognize to help them imagine what a rotting alien corpse smells like... or how a dragon's fiery breath reeks. Keep it simple, but evocative. Perhaps a villain smells of cheap cologne ... or a funeral home smells of old flowers.
I find that after my first draft is finished, it's easier for me to go back in and add the sensory details afterwards, focusing on each scene to make sure all senses are stimulated. As writers we have to make the words "slip into the reader's bloodstream." On the flip side, you can overdo it just as you can underdo it. We have to find the perfect amount of sensory details to stir the reader's emotion without them even realizing it.
I read a suggestion once that said to highlight each scene with differet highlighters: yellow for sight, pink for smells, green for sounds. You should see your pages as a rainbow of color. If you notice more of one color than the others, go back and reevaluate how you can even it out so all senses are stimulated.