The first step is to find a co-founder with complementary competencies. If you can’t sell your idea to anyone smart and qualified so that a person begins to work together with you for free hoping for future profit, then how will you sell it to millions of users?
The second step is to sell your idea to the users pretending that you have a finished product. Not sure where to get customers? Can’t get the cost of clicking the Buy button down so that it fits into the margin of the product? Then you found another good idea that can not make money.
The third step is to describe your non-existing product in a letter to the investor so that they invite you to discuss it in more detail. If you don’t get an answer or you don’t get invited for a talk, you will not get the investment, even if you had the product.
When should you start making the product? Well, somewhere in between the steps above. Or even later.