(1) Talk to your existing users: If your product has a free version, reach out to all of them and make sure you speak with as many of your freemium users as possible. If your product only offers a paid version, again reach out to all of them. Anyone who has already voted with their credit card is a very important person to talk to.
(2) Maximize learning: Understand what are the biggest pain-points you are helping your customers solve. Drill down on their psychographic profiles. Map everything out.
(3) Identify your optimal paid ICP: This is a best guess effort. Do not worry about nailing it 100%. Just make sure it is as close to your current full understanding as possible
(4) Go after them. The HOW is not as important as long as you understand the WHO.