Initially you'll have to go through the process of filling in the online application form. It may or may not involve answering some question (text). Have everything required close at hand to submit it when asked. Second phase is a video recorded interview. They ask 5 or 6 questions and you are recorded through your webcam (in case of a laptop) as you answer them. Can be a bit stressful due to the timer that is obvious and even goes red during the last few seconds. You can review your answers (recordings) after you're done. The third phase is the in=person interview at CERN, Geneva. The most exciting one. I got the chance to visit CERn and even though it was a day full of interviews, I would still opt to go through it once more.