Just as you can exercise to improve your muscles, you can exercise to improve your writing.
The point of writing exercises isn't the exercise, any more than the point of exercising your muscles is lifting weights. Like weight lifting, writing exercises increase your abilities and build capacity you need in real life.
While there are a lot of these exercises, games almost, that writers can do, only about half of them involve actually writing. That's because writing is, at best, only half about writing. The other part of effective writing is seeing. Like a painter, a writer has to be able to see before he or she can reproduce. Often the seeing is as hard as the writing. And like the writing you need to build your capacity to do so with exercises.
There's one exercise I'm particularly fond of builds skill in both seeing and writing. It doesn't take much time, it doesn't require equipment, not even a notebook and you can not only do it anywhere, it's actually better for being done anywhere.
The essence of the game is simple. Describe something or someone in just a sentence or two. It doesn't have to be less than 25 words, but capture the person or the thing in prose while it is in front of you.
This does two things. First, it teaches you to observe and second it tests your ability to put those observations into evocative prose. Additionally, it lets you check your observations while the thing or person you're describing is still in front of you.
Fiction writers are usually very close observers. They have to be. Even if the characters, places and situations are completely made up, they are stronger for being based in real observations.
You should try this game on everything: Objects, scenes, buildings, trees and most of all people.
Remember character counts. With people try for observations that capture something of the inner person. Do they seem happy, sad, preoccupied, self-satisfied, what? Now, what are the physical characteristics that give you that impression? Is there something about the set of the mouth, the slant of the eyes, the way they hold their bodies? What is it exactly?
After a while you'll find this gets a lot easier, often almost automatic. Then you can ring in changes. For example you can do a description of a person that's intended to convey a positive impression. Then you can turn around and create a description of the same person that's negative.
Above all, try for the particular and get away from generalizations. An 'old car' isn't as descriptive as a "dusty 76 Chevy with big patches of gray primer on the rear fender and a skirt of pink Bondo along the bottom."
The purpose isn't to capture these people or things for use in a story. Instead you're honing your skill to capture something and describe it concisely. It isn't easy, but like so many other things, you improve with practice.
And if happens that one day you can pull out bits and pieces of what you've seen and described and use them in your fiction, so much the better.