A sequel is a story that launches from a previous story and tells what came after or as a result of the first story. In the case of the violently graphic short story "Greasy Lake," a sequel would unravel what happens to the narrator and his friends when the ordinary circumstances of daylight life meet the aftermath of a cannabis and alcohol filled night, a night of violence and tragedy.
More specifically, your sequel would tell what happens after they commit the primal crimes, the "Ur-crimes." Your sequel would also need to tell what happens after the psychological shocks of the separate incidents of horror, realization and primitive urge: hot-bloodedly hitting his foe with the tire iron; coming face-to-face with a watery corpse; the attack on the girl. While the story is particularly violent and brutal, with scenes that are intense and graphic, the writing of these scenes comprises only a few lines: 8 to 10 lines for the attack on the girl and the image of their being "dirty, bloody, guilty." Your sequel would maintain the ratio developed in the story, proportioning the amount of space given to psychological introspection versus that given to dirty, bloody guilt.
Finally, your sequel would develop the allusion at the end of the story that suggests an association of his fate with "fence," a symbol for incarceration. At the end of the story, the narrator can only imagine what is facing him, but there is a suggestion of certainty that it will have something to do with fenced-off imprisonment. The sequel would fulfill this fence allusion, an allusion reinforced by the earlier imagined conversation: "'I don't know, the murderer said. Something came over me.'"
Stylistically, your sequel would preserve the elements of craft that develop mood, foreshadow events and reveal character psychology. More specifically, your sequel would continue the poetical references to nature: "eastern half of the sky ... cobalt"; "the birds had begun"; "buds and opening blossoms." Nature references develop the contrast between what will happen in daylight and what happens that night. A critical element of the nature references is the inclusion of harsh descriptors: black, take over, lay slick, sun firing, fence round the perimeter. These harsh words in the heart of a poetical reference to birds, flowers and a dawn sky set the mood and develop foreshadowing, foreshadowing that provides key indicators of what the sequel would reveal as the fate of the narrator and his friends (who seemingly would share the same fate but not necessarily so).
The writing style of the sequel would demand intelligent writing. The narrator uses Biblical, Classical and cultural allusions in his ordinary course of thought: e.g., cultural allusion, "Louisville slugger," a kind of baseball bat. For integrity in his character, which would not change during the confrontation and punishment part of his story (think of Camus' The Stranger)--and they will face punishing consequences since they themselves are walking evidence of horrible deeds as the Bel Air is driven evidence--his mode of personal inner thought needs to stay the same, and his mode of thought indicates a high intelligence and a quality education, this notwithstanding his existential angst-ridden psychological state.
One other stylistic consideration based on the psychology of the narrator strongly suggests the inclusion of flashbacks to horrors, shocks and primitive urges: e.g., flashbacks to the dead man in the water, the blow with the tire iron, the screams of the foxy girl, the wobbling of the stoned girls in stilettos. Note that part of the stylistics of the original story is the use of labels, like "the fox," instead of names, a technique that makes use of Jungian primal archetypes to develop character psychology, while the narrator's psychology is further developed by his stream-of-consciousness musings.
"I contemplated suicide, wondered if I'd need bridgework, scraped the recesses of my brain for some sort of excuse to give my parents .... Then I thought of the dead man. ... and felt the tug of fear, felt the darkness opening inside of me.... Who was he I wondered, this victim of time and circumstance...." (narrator, "Greasy Lake")