As a writer, my biggest advice to you would be to read as much as you can. Find flash fiction, short stories, novellas, and even books and entire series in the genre you want to write, and read them. Pick a story (I write Horror almost exclusively, so for one exercise I chose H. P. Lovecraft's "At the Mountains of Madness") and then read it to completion. Then, read it again while taking notes on everything the author did right that you want to emulate. How long was the longest word? How long were words on average? How many words in their sentences? How many sentences in a paragraph? What about word choice, did you ever have to look up any words? Do you think others would have to look up any words, and is that good or bad? What about the flow, what are the main points of the plot? When did the primary problem in the story show itself, and what earlier problems were there that may have seemed to be the primary problem when they were happening? Ask tons and tons of questions, and seriously look at the answers.
Then, write a lot. Write down 10 concepts for stories you want to write. Make outlines for all 10, and pick at least 5 that seem doable - if all 10 seem doable, pick all 10. Write the first draft of the first one, and then put it down for anywhere from a day to a week. Go back to reading. Come back to it, read it again - is it total shit? The likely answer, surprisingly, will be no, it'll most likely seem salvageable even if it's not as good as you thought it was when you finished it a week earlier. If you can't see how you can salvage it, toss it and move on to idea 2. If you can, show it to people. Post it on here for feedback, show it to friends, family, people you trust to give honest feedback - then take that feedback to heart as you move on and write your second one.
You'll find that by taking writing very seriously, doing intense reading, intense thinking, and intense practicing where you ask yourself, and have others answer, "What is wrong with this story?" You will improve very quickly. Your friends that you show your works to will see a noticeable improvement in your second story over the first - and by the tenth you'll seem like a whole new author, even if you're nowhere near as good as you want to be.
In short, you'll never get better at writing without doing two things: reading, and writing. I'd like to emphasize active reading (taking notes and whatnot), because many people gloss over the importance - it's very frustrating to see a work you want to emulate, and to just flail wildly until you get lucky and hit the vague theme you were going for. By doing your research and thinking hard about it, you'll get ideas on how to craft your writing to fit what you'd like - and then in doing so, you'll find out what you don't like about it, and that's where your style will come from. The synthesis of what you like about your influences, and what you wish they'd changed.
Anyway, that's the strategy I took, and it worked for me. Hope it helps, keep in mind there's nothing that works for absolutely everyone.
PS. On your comment of "It's best to try to practice actually writing comics", all writing is writing. Comics have less writing in them than stories, and your goal should be to write as much as possible. So, for practice, writing stories will give you the most for the shortest amount of time, at least for a while. Eventually you'll want to add in comic practice, but never stop writing stories if you want to improve your pacing and structure.
