Decide on your particular area of study. As the seeker of a Ph.D., you'd already have earned your master's degree, but prior to embarking on your Ph.D. studies, you need to decide if you want to continue in a general direction or on a more specific path of study. As an example, you can get a Ph.D. in biology, a general discipline, or in marine biology, a smaller field.
Pick your future career path. Some Ph.D.s are involved in research, while others are out in the field. A good example of this would be a medical student who decides to earn his Ph.D. Would he prefer to spend his career in a lab, in a more public venue to practice his craft, or perhaps as a teacher, which can be very lucrative.
Find a program that's suited to your needs. There are several different kinds of Ph.D. programs available. Some will be tailored to those who are already working and earning their Ph.D. in their spare time, while others will be intended for full-time students.
Secure financing or grants to fund your Ph.D. Unless you're able to fund the cost of this further education on your own, you'll need to secure student loans or federal grants (or a combination thereof) to continue your studies. Some organizations offer grants to students in particular areas of study. Research those opportunities and explore all methods to get financing for your doctorate studies.
Apply to a Ph.D. program. Ph.D. programs are run just like regular higher education programs; you'll have to subject an application prior to the beginning of the school year. Some Ph.D. programs may require several months advance enrollment and they can be quite competitive, since the class sizes are generally quite small.