Use tracer bullets to find the target. For important requirements and the biggest risks, prioritise coding them first. You're working in the dark, so try to illuminate them ASAP.
Keep working at it until you're on target, and you're now working in the right direction, with all the knowledge you've learnt along the way.
Remember that tracer code is not throwaway code, unlike a prototype. Prototypes generate disposable code. Tracer code is lean but complete code that is the foundation of the system. Prototype to gain reconnaissance and intelligence, and then fire your tracer code in (vaguely) the right direction.