The field of proposal writing is a bit of a misnomer in that no degree grants someone the title of, "proposal writer." It is a discipline that is learned from others already working in the field, studying their methods and seeking grants/private funds for small, and sometimes large organizations. These funds can be quite large, and a skilled proposal writer can be in high demand when the money is on the line.
Becoming a Proposal Writer
Volunteer for a nonprofit organization. These are local organizations that do charitable/political/environmental work and seek no financial gain. Ask questions: Who does your proposal writing? How can I help. Let them know proposal writing is something of interest. Learn from already established professionals working for the given nonprofit. Nonprofit organizations depend on public donations and grants to keep running, so the experience of learning from those well versed in writing proposals will be invaluable going forward.
Write to the task. A proposal writer must convey their client's ideas and concepts in a clear and compelling manner in order to secure the funds or services the proposal is being written for. Ask the client for specifics about the proposal: Who is the proposal being written for? What need will these funds/services being acquired fulfill? Why does your client deserve them over every other applicant?
Research proposal strategies and opportunities. The majority of proposal goals will have an intricate amount of paperwork in order to successfully submit to a given organization. Do the homework and learn these methods on a case-by case-basis, building a knowledge base. "My first foray into the field came when I was a volunteer...for a non-profit organization," writes Beverly A. Browning in her book "How to Become a Grant Writing Consultant." Although she knew nothing about grants, she researched information on writing a proposal and completed it on her own.