What does the phrase "it is up to us to flesh it out" mean?

From the New Oxford American Dictionary:

up to: the duty, responsibility, or choice of (someone): it was up to them to gauge the problem

 

flesh something out: add more details to something that exists only in a draft or outline form: the theorists have fleshed out a variety of scenarios.

So this sentence means (it may not fit exactly, you did not give much context): “it is our responsibility to complete this proposal”