Identify a common problem
There are many problems that need to be solved in this world. The problem has to be a common one, so that enough people need a solution to this problem. And the problem has to be important enough so that people are willing to pay for a solution
Provide a solution to the problem
This is the product development phase. You have to solve a product that actually solves the problem that these people are experiencing.
Find people experiencing the problem
This simply means that you need targeted advertising. Where to find your target audience will depend on the specific problem. Maybe you will find them on search engines. Make sure that your keywords are very specifically targeted at those who experience the problem you want to solve. Or maybe you can find your target audience in certain communities, such as online forums or self help groups.
Prove that your solution works
Once you have found your target audience, they will only be willing to buy your solution once you prove to them that your solution actually works. The proof can be social proof (such as testimonials), or it can consist of examples of the results. If you are selling software that removes red eyes from photos, the proof can consist of before and after pictures.