Gateway API for a Flutter App

I am looking to develop a Gateway API to support a newly-developed Flutter App. Specifically, I am looking to build a REST API. I have a rough idea of what I'm looking for, but I don't have any written specifications yet. As far as authentication needs, I would like to use Basic Authentication. The Freelancer I hire for this project will have the important task of creating a secure and stable API that meets my project goals.