Translate a Vague Creative Idea Into a Specific Project

A vague creative idea stays vague because it has not yet become a decision. What format? What length? What constraint? The moment an idea has specific boundaries, it becomes a project rather than a concept. This finds the one decision that turns your idea into something real and gives you the first steps if you decide to start.

I have a vague creative idea that has been sitting in my head for a while and I want to turn it into something specific enough to actually start.

The idea: [describe it as you would explain it to someone: the image, theme, concept, or feeling at its centre]
Why it has stayed vague: [describe what has kept it from becoming a real project]
What format or medium you are considering: [describe, or say you are not sure]

Help me make this specific: identify the one decision that would turn this idea from a concept into a project, propose a specific scope (length, format, constraint) that would make starting possible rather than optional, and give me the first two things I would need to do if I decided to start this week.