Obtain a PhD degree in English. A degree from an Ivy league school, or another top university, may play into the decision to hire you, but ultimately, frequent publication in scholarly academic journals, as well as teaching, play into whether or not you are awarded tenure.
Apply for a position as a lecturer or a visiting professor. This is not the best route, but it never hurts to try. A lecturer position is often the same as "Instructor", which is usually a non-tenured track position given individuals with expertise in specific areas. Harvard University, for example, has hundreds of 'lecturer/instructor' positions it has awarded to individuals in the greater Boston area; few are tenure track. According to the American Association of University Professors, (AAUP), 65 percent of lecturer positions are non-tenure track. Positions as visiting professors are usually awarded as a temporary position for individuals who have achieved a high degree of success in a specific area, such as a famous author or poet, who becomes a 'visiting' professor for a specific duration, such as one or two academic seasons.
Publish in your area of expertise to build an academic reputation and progress to an assistant professor position. English professors often publish research articles about a certain type of literature. Team up with one or more professors on some projects and produce others on your own. Publish these in scholarly journals.
Apply for an assistant professor position when one become available. Once you get the job, most likely you will be in a tenure-track position, but likely you will be given a probationary period for several years before being granted tenure awarded in a promotion to associate professor. The tenure track is considered the Holy Grail in academic life.
Meet with the English department chair to talk through the tenure process. She can tell you how the tenure application process is specific to your university, and offer any advice. Interview one or two English associate professors if the department chair isn't available.
Submit a tenure application at the end of your probationary period. Follow the directions exactly. If you're not sure about a direction on the application, seek out someone in the department, such as the chair, for clarity.