As Stephen Covey (who sadly died recently) proposed in his classic 1989 book The 7 Habits of Highly Effective People – you should start with the end in mind.

Envision your product before you create your product!
Start by creating your product. By that I don’t mean rolling up your sleeves and starting to work on the production of your product. Nope – start by envisioning your product. In other words: Create your product, before you create it.
That’s the concept in theory, short’n’simple. In practice, it means for example:
Create your sales page before you create the product itself.
Work backwards.
It helps you see what it is that you have to do, to make your product fit what you had in mind. Which features it should have, and more importantly why anyone needs it.
Get it?