This is the opposite of vapourware. By the CTO of Amazon Web Services, back in 2006, an all-time classic for me:
To ensure that a service meets the needs of the customer (and not more than that) we use a process called “Working Backwards” in which you start with your customer and work your way backwards until you get to the minimum set of technology requirements to satisfy what you try to achieve.
Therefore, completely counter-intuitively,
Start by writing the Press Release. Nail it. The press release describes in a simple way what the product does and why it exists – what are the features and benefits. It needs to be very clear and to the point. Writing a press release up front clarifies how the world will see the product – not just how we think about it internally.