How would I find the author's purpose and prove it using text evidence with a simile? In addition, how would I connect this to the theme of the book?

Huckleberry Finn is a satire from the very first word. The book opens with a "Notice" in which the reader is warned that "persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot." This notice is issued "BY ORDER OF THE AUTHOR." Like the story that follows, however, this opening is not to be read literally. The "warning" satirizes similar strongly-worded public orders such as those pinned by farmers to their gates and is signed by GG, the "Chief of Ordnance." The tone is thus set for the novel to come, simultaneously authoritative and slightly ridiculous. But Twain is not finished: after the "Notice" comes a further "Explanation," which introduces a major theme of the story: that of divisions within society, caused by race or class differences and represented by Twain in the novel by varying orthography when writing dialogue. These differences in orthography, the "Explanation" says, are not "haphazard . . . or by guesswork" but have been done "painstakingly." Twain jokes that he gives this explanation in case readers might otherwise think "that these characters were trying to speak alike but not succeeding"; the truth, however, is hidden in the preceding statement that he is representing these dialects "from personal familiarity"—that is, he is personally familiar with the existence of people across different levels of society, like the people in the book, and although he warns readers not to seek a "moral" in the story, at the same time he indicates that it is a representation of how things really are, albeit overblown and satirical.
In terms of identifying a simile to connect the author's purpose to the theme of the book, there are several examples in Huck's dialogue—why not use the simile "sweat like an Injun," as Huck does? The fact that he would compare himself to someone from a culture he is unfamiliar with, and yet feels he has knowledge of, is telling. It is a frequent pattern in Huck's speech, however, to describe one living thing as being like another: people are variously described as being "like a duck" or "like a frog." Huck recognizes things based upon his own understanding of the world, and it is notable that Jim is never described as being "like" anything other than himself: to Huck, he is simply a man, although the patterns of his speech make clear that to the rest of the world, he is perceived very differently because of his race.