Scout learned to read by sitting in Atticus's lap while he read in the evenings and following his moving finger while listening to him read. At some point, the "the lines above Atticus's finger separated into words" (20).
When Miss Caroline--her first grade teacher--discovers that she also knows how to write because she gets bored and begins writing a letter to Dill, she gets in trouble for this, too. (It's a double sin, somehow, because she isn't even printing--she's writing cursive). She says, "Calpurnia was to blame for this" (21). On rainy days, Cal would "scrawl the alphabet" across the top of a tablet, then copy a chapter of the bible. Scout was to copy this as closely as she could. When she did well (which was rare), Cal would give her a treat.
In To Kill A Mockingbird, Scout cannot exactly remember how she learned to read. In reflection, she never remembers not being able to read the hymns in church.
She remembers sitting in her father's lap as he read the paper to her and she followed his finger as he underlined the words for her. She cannot remember exactly when the letters one day separated into words. She was a very inquisitive child so there is no doubt that she learned to read because she was interested in learning to read. She remembers in her own words, "I had stared at them [words] all the evenings in my memory..."
Truly, Scout is wise beyond her years, and she and her father have adult-like conversations. Scout is more mature than many children her age. It could be due to the fact that she is raised mostly by a man. Her mother died when Scout was two. Scout has grown up in a house where her loving father is an intellectual. Her father is constantly reading to her. In reference to community affairs, Atticus talks to his children as if they were adults. He does not try to spare them the truth. He is up front with his children. He is a great teacher.
Also, Scout's housekeeper Calpurnia insists that Scout copy the alphabet in cursive. Calpurnia is like a mother figure in her life, and she disciplines Scout as if she were her own daughter. Scout tends to drive Calpurnia crazy on rainy days so she assigns a writing task to keep Scout busy. Calpurnia rewards Scout for copying a chapter out of the Bible. Scout blames Calpurnia for teaching her to read and write:
Calpurnia was to blame for this... She would set me a writing task by scrawling the alphabet firmly across the top of a tablet, then copying out a chapter of the Bible beneath. If I reproduced her penmanship satisfactorily, she rewarded me...
While Scout probably resented the task of copying chapters from the Bible, no doubt it helped her learn to read.