For designing the UI, real straight forward mocking it up in Sketch. The tough thing though was accounting for the art board size and as I’ve come to learn — is totally dependent on the device you’re optimizing for.
Animations/interactions were done in Principle, and mocking it up in VR was a combination of importing sketch assets onto a spherical background in Photoshop then viewing the comp natively on my device using Mobile VR station. Definitely not a streamlined process by any means, but it worked at the time.
Eventually when we decided to optimize for Rift, we used the same process but different art board dimensions, and had our engineering partners provide a prototyping build created internally that us to throw flat (spherical) comps into to be displayed in the Cv1.
Hope that helps, and let me know if you’ve got any other questions 😎!