Refine your concept as much as possible before you build a minimum viable product (MVP).
Further MVP Iterations
As you get user feedback on your first MVP, you will need to improve it to get to the point where you are satisfied with what your startup has to offer.
A focus on releasing an MVP means that you can potentially avoid lengthy and unnecessary work.
Pre-Launch Model V1
Once you have gone through multiple MVP iterations and tweaked your product based on customer trials, build a pre-launch sales model.