What do you do right before the 'Swagger step' of development?

The benefit of the API design-first approach is a revelation to me. I am trying to incorporate this step into my process and so I would like to learn about software development process from members of this community.

What do you do before involving the swagger editor?

For example: would you have already designed the UI of your app? Created UML diagrams connecting all of your database models? Documented project objectives?

What are the steps before swagger and what are the deliverables of those steps?

Thank you in advance for sharing!