Input: Feedback from customers.
Process: If you do have customers, talk to them and find out why they bought your product over your competitors. Also, talk to other than the traditional decision makers, as the competition factors might be off target from the actual customer needs (f.ex. "overserving"). Also, if possible, find out why someone is buying from your nearest competitors. This is best done as structured interviews with buyers.
You should also map why customers are not buying your product, in order to evaluate alternative strategies to create new segments. If you only interview current customers, the answer would often be "more of the same for a lower price".
The goal is to produce a list of the most desirable features and reasons why customers buy the products. These are your target Unique Selling Propositions (USP).