1.Build the LP - converts?- If yes, move to step 2. If not, keep changing the traffic sources, message, style, features, product till something makes it convert. When it works, move to step 2.
2. Build the website like you have built the product but you haven't - If it converts build the product. If it doesn't, try step 3
3. Build a demo, simple video, all the stuff to show how your product works though you are yet to build the product - still doesn't convert? - Chuck it or pivot. If it works, build the product.

3 parts:
1. Find new sources to gather fresh signups
2. Create a referral program
3. Engage with people who sign up, in a way they talk about you to their network and boost your viral co-eff. This includes exciting newsletters that hit the chord.

Though there are general strategies to all of these which I assume you are aware of, for specific strategies it is important we understand what you sell and your market.