Creating a screenplay for the famous award-winning science-fiction short story "Repent, Harlequin! Said the Ticktockman" by Harlan Ellison would involve several important steps. By the way, a few years ago, with Ellison's blessing, J. Michael Straczynski, the creator of Babylon 5, created a screenplay of the short story, which was in development as of the mid-2010s.
The first step in writing a screenplay of the story would be to get permission to do so from the copyright owner. Harlan Ellison died recently, and presumably his copyrights are now controlled by his wife, although there are undoubtedly agents you'd have to contact first. Ellison was always zealous in protecting his copyrights, and undoubtedly that practice continues with his estate.
It's also imperative that you know how to format a screenplay properly. There are many guides that can teach you how to arrange the margins, font, page numbers, scene headings, and character dialogue. If you want to leave these details to technology, there are several excellent software programs that format scripts automatically.
Usually studios first want to see a synopsis, which summarizes the entire story. In a complete screenplay, the story is broken into acts. The standard number of acts is three, although some longer films have four or even five acts.
"'Repent, Harlequin!' Said the Ticktockman," at least according to the story version in the original written form, would work well in a classic three-act structure. Many screenwriters embellish the basic material of the story, especially if the story is short and contains only the plot essentials, as in this case. How much you would be able to do that would depend on the agreement you make with the copyright owner. In the short story, Ellison makes it clear that he is starting in the middle and not in the beginning.
Now begin in the middle, and later learn the beginning, and the end will take care of itself.
This is perfectly acceptable in a screenplay if the writer so chooses. It means that the film would start with plenty of action and humorous situations as the Harlequin plays his tricks and eludes authorities. Then act 2 would be a flashback showing how the Harlequin became how he is. Act 3, of course, would involve the tragedy at the end, when the Ticktockman deprives the Harlequin of his freedom and ability to think for himself. You could also effectively tell the story chronologically. It's really up to you. Either way, "Repent, Harlequin! Said the Ticktockman" would make an incredibly exciting and dynamic movie.