you're not very specific on what you're already doing, so this might be what you're doing already.
i would say that you should get them to agree on the most basic of components/ideas, commit to it, and then build it. repeat. iteratively design and build features. try and force them to design what is in front of them now, not design the product now for 10 features in the future, or design the entire project before building it out.
i would say that you should get them to agree on the most basic of components/ideas, commit to it, and then build it. repeat. iteratively design and build features. try and force them to design what is in front of them now, not design the product now for 10 features in the future, or design the entire project before building it out.