As others have said, the tips pinned in the discussion at the top of /lit/ are a really great starting point. I definitely recommend taking the time to read them.

Here are some of the guidelines I personally use when writing stories:

Proofread, then proofread, and then make sure to also proofread (I usually go for 5 times, minimum). When you read it, try to only imagine what is actually written, ignoring the details which may only exist in your head.

If you find some part of it boring by the third time, that part probably either needs to be trimmed down, or it needs to have some more spicy details mixed in to recapture interest.

Be prepared to allow the story to take you in an unexpected direction. Accept that something you had planned originally might not actually work well in a particular story, even if it is a good idea for some other story.

Try to have fun. Counter-intuitively, working harder on a story does not always make it better, but if you are enjoying writing (and proofreading) a story, odds are that others will also enjoy reading it.