In To Kill A Mockingbird, hows does Harper Lee create and sustain in Chapter 17 and 18?
This is a bit of a challenge to answer as you have left out a key part of your question - create and sustain what, exactly? However, as you are possibly referring to suspense or dramatic tension, here are a few ideas.
Chapters 17 and 18 are all about the trial and the testimony that is being given. I chapter 17, the prosecuting attorney, Gilmore, is questioning the sheriff, Heck Tate, who paints for us a picture of what he found when he arrived at the Ewel residence on the day in question. We are given an image that we should, as readers, begin to connect with and feel pity for Mayella as a broken and abused woman who may have just been raped and already lives in less than ideal conditions. However, as Atticus begins to cross-examine him, we learn that she did not see a doctor (uncommon in a rape case today, but less uncommon then) as well as another KEY piece of information - the fact that all of her injuries were to the right side of her face. This is a bit of foreshadowing as to what Atticus plans to use in his defense case and should generate suspense for the reader who begins to wonder if things are not as they might appear on the surface. The detailing of the Ewell's living conditions also should plant a seed of doubt in the reader's minds as well as to what really happened on the night in question.
In chapter 18, the trial continues. We see Mayella herself take the stand. Lee makes us feel sorry for her, as she is nothing but a frightened and uneducated "child" (although she is over 18, she is extremely immature socially). Then, Atticus builds his own case - the case of a victim who was not raped but beaten by her own father. He essentially breaks down the prosecutions case, clearing up the significance of the fact that she was injured on the right side of her body - something Tom Robinson could not have done. She breaks down, but does not change her story at all. Instead, she begins a tirade in the courtroom and the prosecution rests leaving Atticus to call his only witness, Tom himself, to the stand.
The build-up in these two chapters is that of a standard courtroom drama. The reader gets to hear the evidence presented just as they would in an actual court of law. Like the jury sitting in the stands, or like Jem and Scout in the balcony, we see each aspect of the night in question revealed to us. As the truth becomes clearer to us, we should begin to side with the defense (if we have not already begun to do so) even though we know that the outcome, given the social situation and racial tensions of the era, will likely not be in Tom's favor.