What does the phrase "it is up to us to ﬂesh it out" mean?
Can you suggest any synonyms?
From the New Oxford American Dictionary:
So this sentence means (it may not fit exactly, you did not give much context): “it is our responsibility to complete this proposal”
"flesh it out" means to add flesh to the body of something. That is to say, fatten it up, make it more substantial, take a bare bones skeleton project/example and make it a fully formed entity.
In short it is using the metaphor of a body, a skeleton, that needs flesh to be complete - to refer to anything that is just the basic structure and needs to be filled in, completed, made whole.