Description transforms telling into showing. It changes report to experience.
In a nutshell, description changes words into the mechanism that helps imagination and memory create emotional experience in the reader. Its transformational property is what makes description valuable.
So, how do you make description perform? Every how to write book and article I know of fails to articulate the process, but the recipe is easy and simple.
As with any recipe, you don't want any ingredient or task that doesn't belong in the mix. So you need to know what outcome you want and include ingredients that are consistent with this outcome. If the outcome is a gut-wrenching scream, stick with Halloween related colors, weather, apparel, and activities. If the outcome is national pride, go with the 4th of July, summer colors, weather, and activities. We're talking congruence NOT Christmas in July. Everyone has seen a two-headed snake, but they work best in the appropriate time and season, say Halloween. Don't startle the reader unless it's the outcome you're after. Twain suggested that there is a world of difference between lightning and lightning bugs. Use the right words.
Move from general to specific, descend the ladder of abstraction with your descriptors. Avoid the example of this old, ribald saw, "I used to kiss her on her sweet red lips, but it's all over now.' Do the IT'S ALL OVER NOW part first, then move to her lips or wherever. Pan the crowd and focus on the individual. See the forest, then the trees. Study a few hypnotic trance inductions to get the hang of how you work from the general to a focal point. Don't obsess about crates: When my great grandfather died the local newspaper featured an article about his library. The library contained 2500 books, many of them rare and expensive, but not one word in the article spoke of the books; the reporter was obsessed with the crate the books were transported in, and every word of the article described or opined about the crate.
Adjectives and adverbs aren't generally bonafide description, though Hemingway created some magic using adjectives behind subjects, and I know of one master poet who pulled it off, too. But Ernest was the exception and a genius. Adjectives and adverbs are commands from author to reader about what the reader should sense and feel; they're impositions. If you plan to swat the reader with your will, smack her with candy apple red rather than fire engine red NOT really red or bright red.'
In conclusion, a story isn't supposed to be schizophrenic word salad or a bag of maniacal digressions, tangential points, and loose associations. Such fare worked for Thomas Wolfe but usually lead the reader through the looking-glass and confuse them beyond redemption. Stick to the subject, stick to the path, and avoid chasing rainbows to describe.