Science policy involves a mixture of writing, advocacy, and research to communicate scientific findings to the people who create policy.
This article from ASBMB sums it up: "What Is Science Policy?"
From "Paths to Science Policy" by James Austin:
"But if you're already in science, the best advice is probably this: Finish your Ph.D. and, as you're writing your dissertation, work on refining your policy-career objectives. Bone up on the areas of policy you're interested in, and home in on a few key questions of contemporary relevance. Consider a policy fellowship, like the ones AAAS offers. Engage people working in the field; these days, with e-mail, they usually aren't too difficult to contact. Respect their time, but don't be shy. Develop a dialog and a professional network. Especially if you have a strong publication record, your status as a scientist makes you relevant to the debate from the outset."
Phd required: recommended