Start by writing out your entire story with pen and paper. This is important. Do not start your first draft on a computer.
Begin typing it up on your computer. As you do this make corrections and revisions while typing, however small. Shorten or lengthen sentences, take out unnecessary words, etc. If you do this by the time you have it fully typed up you’ll already have completed a second draft of your story.
Print out your story and begin editing it by hand (that is, with a pen). Go over it line by line, word by word. Think about whether it is necessary to begin the story where you have and if you were to take out the beginning paragraph(s) if it would fundamentally change the story. Check if you have to slow some passages down and add more concrete details.
Incorporate the changes that you’ve made. If additional edits/revisions come to mind, go ahead and do it. Nothing is set and stone and can always be undone later.
Print out your story. Read it over.
Consider that if you’re having trouble thinking of things to change, read the story backwards, one line at a time. This helps you focus in on the actual words and sentence structure that you’re using because it pulls you out of the natural flow and rhythm of the piece, allowing you to see it in more technical terms.
Read it aloud to someone if you feel more work still needs to be done. Have someone else read it. This can provide you with valuable feedback in respect to what is and is not working.