The standard route to becoming a lawyer is to take the bar exam administered by the state the person wants to practice in. Law school prepares individuals with the knowledge to pass the bar exam and therefore practice as a lawyer. In some states there is a way to circumvent this method.
Take the example of Jennifer O’Brien a California lawyer who never attended law school. She worked for 15 years as a legal assistant to lawyer. She took the exam for the first time this summer and in November learned she passed. She studied law under the mentorship of her employer and will work in their law office as an attorney.