Brainstorm about the topic or piece you will be analyzing. Write down the item's tone, argument, approach and any other key factors to the document. Consider questions such as: Who was this document written for, how is the author trying to convince the audience, and what are the author's goals? Think about the items that would be appealing to discuss in your analysis.
Condense the brainstorm into the main argument you would like to posit in your rhetorical analysis. Often, the argument will include why the author was, or was not, successful in stating her case and whether her tone and style was effective.
Compose a strong, structured thesis statement reflecting the main argument you will pursue in the rhetorical analysis. This statement will include the position you have taken on the author's style, tone and effectiveness. Write a few different thesis statements, playing with various styles and tones. Practice will create the strongest and best thesis statement.
Revise the thesis statement if you find that the argument of the rest of the rhetorical analysis changes as you write it. The thesis statement must reflect and represent what the rest of the analysis will argue.