Read research interests and publications of professors that seem compatible with your interests. Send these professors an introductory letter and say who you are, what your intentions are, and request information about that professor's specific research program as will as recent reprints of their articles. You may want to include a copy of your resume. An advocate for you on the admissions committee will be very helpful. Ask three professors who know you and your interests well to write you a recommendation.
Recommendations are very important.
Write to NSF for a Predoctoral fellowship application. Send completed applications for admission and financial aid. Essays should be clear and well-written. The essay should include the area you are interested in, why the specific program is attractive to you, what you have done in the past that is relevant, and the significance of what you did. Cell, Molecular, and Developmental.