For all intents and purposes you don't. People that already have substantial followings can drive traffic for you. If you are a normal person, you can grind out

For all intents and purposes you don't. People that already have substantial followings can drive traffic for you. If you are a normal person, you can grind out signups, but those people will forget who are you by the time you launch. So, what's a person to do? Well, build a product worth using. Get a few people using it and enable them to spread the word for you. Good luck!