When approaching this creative assignment, it would be useful to note Dally's perspective in a few lines as a prewriting exercise. Here are some ideas to get you started.
First of all, consider where Dally comes from: the street gangs of New York. Somehow, Dally ended up in Oklahoma, alone, having been in and out of some sort of incarceration since he was ten years old. Just his background alone explains his bitterness, but what kinds of specific events might you be able to imagine to illustrate his character? These events could form the main story line of your writing if you chose to focus on Dally's experience through flashbacks or memories.
Another idea might involve Dally's relationship with Johnny. What is it specifically about Johnny that enables Dally to feel tenderness and trust? Does Johnny remind him of someone who had a positive influence on him? Going deep into this relationship could make for a very compelling installment from Dally's point of view.
Finally, what goes on in Dally's mind in the moments before he dies? What images and individuals are in his head, and what might he be hoping for with his death? These questions allow you, and your reader, to explore the complex inner workings of a very complex character from The Outsiders, so let your imagination take over (just remember to stay consistent with the textual evidence from the novel itself). Good luck!