How did Bob Ewell attack Atticus?

Bob Ewell despises Atticus for embarrassing him and presumably ruining his reputation during Tom Robinson's trial. During the trial, Atticus proved that Bob Ewell viciously assaulted his daughter after witnessing her kiss Tom Robinson. Following the trial,towards the end of chapter 22, Bob Ewell assaults Atticus by spitting in his face outside of the post office. In chapter 23, Scout recalls Miss Stephanie Crawford's story about Bob Ewell's confrontation with Atticus. According to Miss Stephanie, Bob Ewell spit in Atticus's face, challenged him to a fight, and proceeded to call him derogatory names. Despite Bob Ewell's aggression and hostility, Atticus refused to fight and simply wiped the spit off of his face before walking away.
After the confrontation, the children fear for their father's life, but he assures them that Bob got everything negative out of his system that day he confronted him outside of the post office. Unfortunately, Atticus dramatically underestimates Bob's wicked nature. Following the Maycomb Halloween festival, Bob Ewell seeks revenge on Atticus by attacking his innocent, defenseless children.

Bob Ewell's absolutely furious with Atticus for humiliating him on the witness stand during the trial of Tom Robinson. Atticus expertly took apart Bob's shaky testimony, exposing him to be the ignorant, lying racist that he is. Unfortunately, in a deeply prejudiced town like Maycomb, that wasn't enough to secure Tom's acquittal. But it was enough to thwart Bob's plan to make himself look like a hero in the eyes of the townsfolk.
So Bob's determined to get his own back on Atticus. He confronts him in the street one day and spits right in his face. Not only that, but he openly challenges Atticus to a fight. As we might expect, Atticus bears this unpleasant ordeal with considerable dignity. He always tries to put himself in other people's shoes and so understands just where Bob's coming from. He doesn't rise to the bait, even when Bob threatens to get him if it takes the rest of his life. Atticus figures that if Bob's spitting in his face and threatening him with violence means one less beating for Mayella then it's worth it in the end.