The first thing we (The PM, myself, and the backend developer) did was to hop on a discovery session so we could clearly understand the feature we were trying to build, and then we took a look at what we had available so we could come up with ideas within that constraints, we figured out the APIs that were ready as we wanted to build this product rapidly (it was experimental), once we were all on the same page, we collaborated in creating the flow for the product and then heading straight into the design.