Find the right funding agency. Know agency interests, culture, and style. Use the phone. Call a program officer, briefly summarize your idea, and and prepare specific questions. Take the program officers's advice very seriously, but exercise your own best judgment.
Some agencies are more directive than others.
Use a journalistic writing style. Also use bullets, lists, outlines, diagrams, tables. Don't obsess on any topic, even if important. Follow guidelines to the letter. Match headings in the proposal to headings in the guidelines so the reader doesn't have to hunt for needed information.