People listen up. Or gather 'round. Either way, I have something important to tell you.
To "flesh out" an idea is to add meat to the bones of the skeleton. It is an apt metaphor.
To "flush out" an idea is to grab a newspaper and head to the bathroom. And maybe eat some more fiber. It is an apt metaphor only if you hate the idea and the person who came up with it.
Many people think these are "homophones". They are not. They do no share the same pronunciation or, as we've established above, the same meaning.
That is all.