The niche and loads first, then catalogue hardware, and only then the carcass.
Take niche dimensions from measurements, drawings or photos
Choose hardware for weight and size: hinges, runners, lighting rails
Build the carcass from real thicknesses and joints: 45° mitres, hidden brackets, mounting plates
Check that it fits into place, that doors open, and the model for interference
Issue drawings with sections from the model and the hardware list