Snowflake Pro

The Snowflake method asks you to define a one-sentence summary and then expand that to a Three-Act Structure.

That takes some thinking. It forces you to really understand your own story.
Several of the steps in the Snowflake method ask you to define your characters and then to get to know them -- really well.
Use Snowflake Pro to get a running start on your proposal. And then use all the handy tips Snowflake Pro embeds in the proposal to guide you in polishing it to a high sheen.