Ideally you start working on that before or contemporaneous with building the thing; it informs what you build and how.
It's like "I have a pretty good idea how to sell project management software. How do I make it?"
"Well let's start somewhere tractable then: how does one Javascript?"
"We sell X with pricing page here but we've begun to get indications that it's being used in true enterprises and I've been asked for an RFP. How do we even think through that?"