How do you do "sell first, build second"? - I will not promote

I get the general idea of the "sell first, build second" principle but I'd like to get some inputs from people who actually made that happen as it seems to be as the best approach to start a startup. So far, I've always built the product, at least the rough 1st version, but would love to sell my next idea first, and then spend more time building it. I will not promote