First was a simple test on HackerRank, followed by a phone interview with two employees. During that interview, there was another simple online test in real time. Just basic HTML & CSS. Easy.
About a week later I got invited to Amsterdam for an in-person interview.
HR manager was very helpful and friendly and assisted during the entire time, gave some valuable bits of advice about the interview process. I was asked to prepare 3 things I'd change on the Booking.com website, why and how I would measure the results.
They booked the flights and hotel room for me. All was smooth and well organized.
The interview was divided into 3 rounds with different employees. One technical, second more focused on your ability to think and solve problems, and the final one with lead designer.
Everybody who I met at Booking.com during my visit was friendly and nice people.