After the initial consultation, we would start with a high-level API design, splitting the project into workable modules, with each module individually assessed for testability (unit-tests, and/or integration tests).
We would then develop the core components, building up to a complete working system, utilising agile development practices.
We have had years of experience developing user interfaces, and know what will work, and how to make the interface intuitive.
We can prototype user interfaces, allowing you to investigate options with the design and experiment with ideas.