I need to write a thesis statement about Cash Bundren, in As I Lay Dying, by William Faulkner How would I go about writing the thesis statement?

In addition to the superb answer above, you may want to focus on the style of Cash's narrations, his tools, and their relationship to his mother.  Cash's duty toward his mother is utilitarian and unemotional: he wants to please his mother with things (making the coffin), and even after she is dead he regards her as a thing, something made, whose function is to be put in its proper place, like one of his tools.

In terms of narrative style, Cash is very mathematic.  He makes lists and references angles.  The most notable is the bevel, which is a 45-degree edge.  Geometry is important in As I Lay Dying: Anse is vertical, always standing (he hates things flat: the road); Addie is horizontal, dead in her coffin.  So, Cash is somewhere in the middle: half alive, half dead.  In this tragi-comedy, Cash walks a tightrope between the two, nearly dying twice: jumping into the river to save his mother's coffin and suffering from the complications of having his leg set in cement.

Each of Cash's tool represent a member of the family. (See the essay link below for details).   Addie is the sawset; Darl is the plane; Dewey Dell is the plumb line; Vardaman is the saw; Anse is the rule; Cash is the square:

In his essay "Faulkner's As I Lay Dying," Tim Poland posits that Cash's “entire identity is located in the fact that he is a carpenter,” and that “[h]is sole means  of expression is through his carpenter skills”. Poland notes that when Cash loses his tools to  the flooding river, he simultaneously loses consciousness and the ability to speak. Thus, Poland links the  loss of Cash’s tools to the “submersion of Cash’s selfhood”. As Poland observes, it is not until the tools are found that Cash regains both consciousness and speech.