Earning a Ph.D. is a process similar to earning a Master of Business Administration. Your MBA can make you a more competitive applicant for a Ph.D. program, but you shouldn't count on it, because the skills required for MBA and Ph.D. candidates are different. A Ph.D. requires commitment to research and the ability to work independently for, possibly, several years.
Approach your Ph.D. application process like a job application. Ph.D.s differ from other programs in that they are generally hiring you as a research assistant/Ph.D. candidate rather than having you pay to study like you would for a medical degree or as you likely did for your MBA. A Ph.D. candidate works as a teacher's assistant or research assistant and often does not pay tuition; his job is to be a student. Ph.D. students attend classes and contribute to the university through research, working as an assistant, and, of course, working on a thesis.
Focus on your research experience on your application. Your extracurriculars and hobbies are not as important for these applications as they are for other graduate school applications; Ph.D. committees are almost exclusively interested in what you can do for them in terms of research.
Assemble letters of recommendation from academics that can attest to your researching and writing abilities. Seek letters from professors who have conducted research in your desired field of study.
Focus on your statement of purpose when you send your application. This is what the committee will be looking at the most. Remember to "show" rather than "tell" them how you have researched in the past and indicate what you can do in the future.
Attend classes during your first one or two years as if you were in a standard program. The first couple years of a Ph.D. are actually a master's degree. When you finish these classes, you will be awarded a master's degree and move on to your doctoral work.
Use the first two years of studying to refine your reading and come up with a topic. Look for areas where gaps in the discourse exist that you could potentially fill.
Choose an adviser who specializes in the topic you choose. Choose carefully, as you will be working with this professor for the next few years.
Conduct research and write a thesis. This can take years, as the thesis is a well-researched piece of work.