Your core action is the thing people do in your product to deal with the problem they signed up to solve.

A clear understanding of your problem comes from listening to your users and understanding how they think about it. Find the right level of abstraction. You've found the sweet spot when you can explain the problem using the words your customers use.

Then you have to deliver on your promise. Does your thing actually do what it claims to do?

The problem is the trigger. Your core action is the response. Delivering on your promise is the reward. These three ingredients are everything you need to build a habit.

When the problem comes up next, if someone tries your solution and you deliver, without all the hassle, people are going to remember you. The next time it comes up, guess who they are going to think of. Solve the problem a third time and you're well on your way to forming a habit.

This is how all habits start. These three ingredients. You can reinforce this habit with notifications and ad campaigns and new features and lalala...but the problem has to exist, your core action should be clear, and you must be able to deliver on your promise.