One approach is to go with at it in a hierarchical fashion.
Write a simple outline for the whole story. Put only the important stuff, the things that affect the rest.
Then, dig down into one part of it. Flesh out one level for this part. This will force you do design the settings, the characters, the motivations, what happened before that you must state, what might happen later that you allude to. A prototype there will/may convince you/inverstors that the whole is of interest.
Then just go back to the high-level story, improve it a bit, and budget allowing, dig down into another part.
That way, you avoid useless work, you remain agile, allowing you to adapt to inevitable discoveries (this character doesn't work, I need this type of location/setting, people would rather see xyz).
Best of luck with the project!