As the UX part was already defined the process in this case started with the UI Design:

With the wireframes on the table and the whole UX research done the first step was to see how the client is using their corporative branding and how this can be applied to the app.
The client decided to use Android Material Design as a base to reduce costs, so there was also a research on the best components to use.

Having Material Design as a reference and CaixaBank branding it was the moment to start designing the mock-ups from the already approved wireframes.

With all the mock-ups done the next step was to create a prototype for the client to showcase and give feedback and also to deliver it to the developers. For that InVision was the tool selected for the easy communication between all the parts involved in the process.

With the InVision prototype created it was the moment to present the app to the client and get the final approval. In this case the presentation was at the clients headquarters with a final feedback session where the last approval was given.

Working in an Agile method the implementation of the code was already done on going and some of the already approved screens where finalized. But with the last feedback session and a final version of the InVision prototype and the measurements and the exportable assets selected the developers where ready to finalize the app.
Afterwards a last iteration with design approval was done to ensure that the design and the app looked exactly the same.