Different publications of Harper Lee's To Kill a Mockingbird may have different page numbers, but the scene in question is found in chapter 28. This is the chapter where Jem and Scout are in danger because Bob Ewell follows them home after the Halloween festivities at the school. The children stay behind after everyone leaves because Scout is embarrassed for having missed her cue during Mrs. Merriweather's presentation of Maycomb's history. As a result, Scout forgets her shoes and has to walk home with bare feet and still dressed in her ham costume. Fortunately, she has her big brother, Jem, with her and he hears someone following them. Scout describes being in danger when they hear the stranger's footsteps:

"Our company shuffled and dragged his feet, as if wearing heavy shoes. Whoever it was wore thick cotton pants; what I thought were trees rustling was the soft swish of cotton on cotton, wheek, wheek, with every step" (261).

Soon after this passage, Jem tells Scout to run because the sound of slow footsteps turn to fast-paced running towards them. Everything happens pretty quickly once Ewell attacks the children; and because of Scout's costume, she can only perceive what is going on through sound, mostly. The description of the attack takes about three paragraphs. The full description of Bob Ewell's attack is on page 262 in the Warner Books publication. If you don't have that edition, start in chapter 28 to find the scene.