1. Identify your target customer first (not the product or the market!)
2. Interview your target customers. Take notes. Find the unfulfilled need.
3. Pitch your solution to those customers and have them sign a Letter Of Intent.
The really powerful piece here is the LOI. It stated "I, Company A, intend to purchase Product Z for $XX,000. This is a non-binding commitment and may be terminated at any time."
Even though the LOI was explicitly non-binding and had no legal weight, they found that the cultural weight of physically signing a piece of paper was enough to scare off non-customers and validate true intent to purchase. Once they had 10 signed LOIs they built the product. All the signed LOIs ultimately converted into paid customers (which is rather amazing).