Of all the things that a writer is required to achieve in their story, the one that's probably most difficult, and yet at the same time most important is scene description. This can include action, imagery, expressions, and so much more. The biggest reason behind this is that some things need to be succinct in their description, and others must be played out in detail, covering every aspect necessary to properly build the picture.
And building a picture is exactly what you're after. Think of your words as a brush that is used to paint a picture on the canvas of your reader's mind. If you use the wrong words, or too many, maybe too few, or anything like that and you risk taking your masterpiece and turning it into something akin to the splatterings of a three year old. As any parent knows, three year old don't paint. They just create a multi-colored path of devastation.
So if your scenery descriptions are off, or outright wrong, that's exactly what you'll do, and as a writer, that's the last thing you want. So how do you improve your prose? Well, how would a painter improve their skills? The first and obvious answer is to practice, practice, practice. Oh, yeah, and practice. You can't get better if you're not willing to write, and do it a LOT. The nice thing about writing in volume is you gain experience as you're doing it, and if you're always challenging yourself, you will always get better.
Now, as per the title, it's time for the exercise that will improve your writing skills. Step one. Find a picture, any picture, sit down with it, and describe it in words. Not as you'd be expressing this to a friend (ie, "it's a yellow flower") but rather in a way that seems somewhat poetic. Example:
"The golden yellow petals of the Daffodil plant, covered in morning dew, sparkle like diamond studded slivers of gold in the cool of the dawning day."
You can do the same thing with actions. For example, if someone was looking at the flower, for example a character named Sarah, you could change your description to something like this:
"Sarah bent down and gently caressed the golden yellow petals of the Daffodil plant. The cool morning dew that covered the brilliant yellow flower tickled her fingers and made the leaves seem to giggle with joy."
These are two methods you can use to describe the same scene, one with just a description of the scenery, and one with a paired action from a character who is interacting with that scene. Now, the other thing you have to remember when describing a scene is don't get too carried away. There's doing things the right way, and then the Tolkien way. I use him as an example because he would take pages just to describe something that should have only taken a paragraph at most to describe.
Terry Goodkind was another author infamous for taking far too much time describing scenes. If you'd taken his 12 book "Wheel of Time" series and trimmed all the extra fluff out of them (over described scenes, etc) the entire series could be condensed down into one novel. So always remember to only do enough, but not too much when writing. But what is considered enough?
Well, that's a question that has a rather arbitrary answer that depends on the writer and the story. So how would one determine if they've written too much, or not enough? There are a series of rules that I use for myself which are drawn from my interactions with other more seasoned authors, that seem to work well. At least from my own experience. They are as follows:
1. What are you describing? IE, is this action, an opening scene, a bit of history, character bio, etc. In short, what you're describing goes a long ways towards determining how you describe it.
2. Is it relevant to the story? A LOT of the world you have in your head won't find its way into your book. Mostly because it's not relevant to the story at hand. For example, you might have a character's outfit figured out down to the very stitches. But for the story itself the only relevant facts you might is that they're wearing a blue shirt with red buttons because that's all that's relevant to that scene.
3. Have you told enough? Yes, there are actually times when you haven't told enough about a scene, event, or character. When you fix this, it's called "fleshing out" and involves telling more about what's happening or where someone is in order to better build the scene or the suspense.
4. Have you told too much? Again, like the last question this goes back to #2. There is a point in every scene where enough has been said and nothing more need be added to improve on it. In some cases that extra can take away from what you're trying to achieve. Learning these balance points takes practice and lots of feedback, but in time can be learned.
5. Has it been critiqued? Anytime you're writing something it's a good idea to get it critiqued. For the initial rough draft a friend or family member will do well enough to ensure that the basics are covered and the story is good enough. (FYI, be sure to pick someone who will be bluntly honest with you and not just be a yes man, because only those who are properly critical of your work will actually help you improve your craft) For later versions it's a good idea to hunt down more professional feedback, or someone who is a complete or general stranger.
6. How critical are you of yourself? The sign of a good writer is someone who is self critical. You're always trying to improve your craft, improve what you've written, and your bar of achievement is a moving mark, always one step ahead of where you are now. It's one thing to be overly critical of your work, but at the same time having some degree of self consciousness in your writing is a healthy thing. It will always push you to be a better writer.
Writing is an art, and like any art it's a perishable skill. But it is also one that can be endlessly improved. But you have to be willing to make those improvements and never settle for "good enough". So dig in, write, explore, experiment, and find out where your strengths and weaknesses are. Once you know those you'll be able to improve your craft to incredible heights. :)