Where in the book does Bob Ewell go into the woods to drink?

During Atticus's cross-examination of Mayella in chapter 18, it is revealed that Bob Ewell spends the money from his relief checks on alcohol and travels into the swamp for days at a time to drink by himself. While Bob Ewell is drinking in the woods, Mayella stays at home, taking care of her siblings by herself. As part of his cross-examination, Atticus builds his case by presenting Mayella's terrible home life for the jury to consider. The jury listens as Mayella elaborates on how her father neglects the entire family and even becomes violent at times. Bob Ewell is by far the most despicable citizen in Maycomb, and Atticus tells Scout in chapter three that the authorities even allow him to hunt out of season so that his children do not starve. Instead of using his relief checks to feed his family, Bob prefers to spend it on green whiskey and drink in the forest alone. After Mayella describes her unfortunate home life in chapter 18, Atticus asks her if she has any friends, which offends Mayella and makes Scout realize that she must be the loneliest person in the world.

In Chapter 18 of To Kill a Mockingbird Mayella Ewell takes the witness stand in the trial of Tom Robinson. Atticus is examining Mayella with the intention of trying to build up a picture of the Ewells' troubled home life. This is part of his strategy to undermine the credibility of Mayella's testimony. Atticus expertly proceeds to coax all kinds of information from Mayella that gives the jury a pretty detailed account of what life is like in the Ewell home. The jury discovers, among other things, that Bob Ewell's welfare check was insufficient to feed the family, and in any case he probably spent most of it on booze. He'd often head out to the swamp to drink for days on end, then come home sick. The information Mayella provides on the witness stand doesn't simply undermine her credibility, it gives us a greater insight into just what kind of a man Bob Ewell really is.