Several stages, first a written test (math, statistics, programming), then video conference with members of their research team. Finally interviews in person both with researchers and management. Atmosphere is friendly, most of the questions asked are brainteasers, they also check basic knowledge in the aforementioned fields. They ask you about recent events in finance you might have read about in the news.

Written test first and then if successful oral discussions. There are further rounds if successful. The written test can be taken from pretty much everywhere but I took it from London.