You need to use descriptions of Curley’s wife, things she says, and inferences you make about her to create an original backstory for her since she dies before the end of the novel.
When you are writing an original story based on a character from a book, you are telling a story like any other. You just need to base your story on the details the author already gave you and add your own. You will use the details you have to make reasonable extrapolations for the new details.
All stories have characters, a setting, and a plot. If you are using Of Mice and Men, you will use the ranch and the 1930’s as your setting. Since Curley’s wife is your character, you can decide that you want to base the story on her backstory, possibly describing what happened to her before she met her husband, Curley.
In order to describe Curley’s wife, you will want to look at her description in the book. She does not have the respect of the men, or her husband. She is described as having “the eye,” meaning that she might cheat on her husband because her eye wanders to other men. This is why George worries that she is a “tart,” a trouble maker among men. It is important that you remain faithful to her physical description.
She had full, rouged lips and wide-spaced eyes, heavily made up. Her fingernails were red. Her hair hung in little rolled clusters, like sausages. (Ch. 2)
If you are going to write about Curley’s wife, you have to determine if the men’s description of Curley’s wife is accurate or not. You will also need to fully explore her motivations. It is clear that she is a lonely woman. She complains about how everyone goes into town, and leaves her alone.
"-Sat'iday night. Ever'body out doin' som'pin'. Ever'body! An' what am I doin'? Standin' here talkin' to a bunch of bindle stiffs a nigger an' a dum-dum and a lousy ol' sheep- an' likin' it because they ain't nobody else." (Ch. 4)
Since Curley’s wife dies before the end of the book, you might use the descriptions she gives of the man who says he was going to put her in pictures (movies) as an example to describe her life before she met Curley. You could also describe what was going on right before the time George and Lennie came to the ranch, since she and Curley had just been married. You also might retell the events of the book as we know them from her perspective. You might want to give her a name, since Steinbeck emphasizes her isolation by never giving her one. You can give her one, and give her a voice at the same time.