One metaphor can be found in Chapter 10 of Harper Lee's To Kill a Mockingbird when Miss Maudie says to Atticus, "Atticus, you are a devil from hell." Since Atticus is not literally a devil, we know Miss Maudie is comparing Atticus to a devil to make a point, thereby creating a metaphor. Miss Maudie says this when Atticus comes home for lunch to find that Scout had built herself a barricade in the front lawn from which she was crouching behind to aim her air rifle at Miss Maudie as she bent over to tend to her azaleas. Atticus goes across the street to inform Miss Maudie that she was in "considerable peril" to which Miss Maudie replies by calling him a "devil from hell."
Normally, the idiom "from hell" is used to speak of anything significantly unpleasant or horrible, like something from hell. However, Miss Maudie is using it to make light of the fact that Atticus was inevitably looking at her rear end and to thank him for the warning, similarly to calling him an angel. Scout later thinks to herself that she wishes her father were a "devil from hell" because she wishes he was more adventurous and different from others.
We find a second metaphor in Chapter 11 when Scout narrates the children's experiences with Mrs. Dubose. By the time we reach this chapter, the children have become brave enough to venture into town by themselves, which forces them to pass Mrs. Dubose's house, the meanest old woman in the neighborhood. Each time they pass her house, she showers them with criticisms. Scout uses the following to describe their interactions with Mrs. Dubose:
If she was on the porch when we passed, we would be raked by her wrathful gaze, subjected to ruthless interrogation regarding our behavior, and given a melancholy prediction on what we would amount to when we grew up which was always nothing. (Ch. 11)
In this passage, Scout uses the metaphor "raked by her wrathful gaze" to compare Mrs. Dubose's eyes, with their evil, critical stare, to a garden rake. In other words, in Scout's view, being looked at by Mrs. Dubose was the equivalent of dirt being raked by a garden rake. Since rakes leave such deep indentations in the earth, if earth had feelings, we might assume that being raked would be considered a very painful process. Hence, Scout is using her metaphor to describe being looked at by Mrs. Dubose as a very painful process.