Submit an online application, with as much detail as possible, and answer some questions about your interest in cern and the thing you would like to do . Three very good reference letters from top individuals are required. After the committee decided, I go contacted from the Hr that setup the interview with the supervisor. I was given a description of the project and a asked about previous experience and goals. A couple of technology related questions . The interview took quite some time but HR was very helpful at their own pace.