A work may be nominated by an Active, Lifetime Active, Associate or Lifetime Associate member (with no fiduciary interest–which means, not the writer, not the editor, the agent, publicist,) during the nomination period, which runs November 15 of the eligibility year through February 15 of the following year. An author doesn’t have to be a member of SFWA in order for her work to be considered. The six works in each category with the greatest number of nominations become the finalists for the Nebula Awards for that year.
The best way to have a work considered is to make sure that it is available to be read by SFWA members; SFWA members can put a link to an electronic version of a work in the Fiction area on the SFWA forum. Non-members can ask a member or the Nebula Awards Commissioner to post a link or document to the Forum. After that, the work and its writer are in the hands of the Fates