Obtain a bachelor's degree in the field in which you intend to pursue your PhD. It is possible to get accepted into a PhD program by having a bachelor's degree in another field, but you may face two significant challenges. First, you will have to justify your application for admission in your personal statement or letter of intent. The admission committee will want to know why they should choose you over another student who may have a background in the field already. Second, you may have to take various prerequisite courses to be be considered for the program. Your best bet is to remain consistent from your undergraduate to graduate education, unless you simply cannot fathom remaining in your original field of study.
Apply for admission to programs with scholars whose research interests closely align with yours. As a research-oriented degree, a PhD is primarily geared toward the completion of your doctoral dissertation, which is is a substantial research project that, for many scholars, later gets published as a book. You will likely work with a faculty mentor throughout your program of study, so you need to choose a mentor at a program known for research in your particular area of interest.
Complete all required seminars and coursework as quickly as possible. For most students this will take about two to three years. During that time, you will be gaining the necessary knowledge base to pass your comprehensive exams and starting the nascent stages of your dissertation research. Read everything that you can in your field during this time. Your exams can cover anything from your field of study.
Pass your comprehensive examinations and begin working on your dissertation. Your dissertation can take years to complete. Many PhD candidates take teaching positions while working on their PhD because of the extended length of time it often takes to complete the degree. When your dissertation is complete, you may be required to defend your conclusions in front of a faculty committee. When you have successfully done that, you will be awarded your PhD.