I applied online through their careers site. I got a message from a recruiter about a week later asking me to take an online logical thinking test, that consisted basically of: given three patterns, what would be the fourth one.
Over the course of the following days, I got three interviews on Skype with different engineers from Klarna. The interviewing process was actually quite smooth and the people involved were really nice and considerate.
Though I got no offer, their treatment and the process itself were really good.