It's unclear from your question whether you need to target 8–10 sentences total or 8–10 sentences per character. I'll focus on helping you remember the key points of testimony for each of these characters, and then you can combine this with your own knowledge of the book to meet the length requirements.
Heck Tate: Bob Ewell came by on the evening of November 21, telling Heck that some "nigger'd raped his girl." Heck accompanied Bob to the Ewell residence, and he found Mayella beaten. She told him that Tom Robinson had hurt her. The evidence he provides that is key to the outcome is that Mayella had visible finger marks all the way around her throat and that she was beaten on the right side of her face.
Bob Ewell: Bob doesn't present as a credible witness from the beginning, making unsavory remarks about Mayella's mother and generally being crude and offensive. He testifies that he heard Mayella's screams and ran to a window, where he saw Tom Robinson raping her. He then ran to get Heck Tate. Upon cross-examination, he also testifies that he did not seek medical attention for Mayella. When Atticus asks Bob to sign a paper, it is noted that Bob is left-handed.
Mayella Ewell: Mayella testifies that she paid Tom to bust a "chiffarobe" for her, and when she went inside to get the money for him, Tom followed her inside and started beating her. She testifies that her father treats her okay "except when..." and then changes her course, testifying instead that he has never laid a hand on her. She testifies that Tom grabbed her around the throat, choking her, and then managed to hit her left eye with his right fist because it "glanced" as she ducked.
Tom Robinson: Tom's left arm is completely useless to him; he has to use his right hand to place it on the Bible to be sworn in. He testifies that he busted up the chiffarobe for Mayella over a year before and did so for free. He returned lots of times because Mayella always seemed to need his help with something. On the night in question, Mayella asked him to fix their door because fall was approaching. All the other children were gone because she had saved for a year to buy all of them an ice cream. Suddenly, Mayella "jumped on" him and started kissing him, saying that she'd never kissed a man before. Tom was trying to escape when Bob looked through the window, and he threatened to kill his daughter.