First you subscribe in the online database. If you succeed to enter the BlueBook, you have to lobby for traineeship positions or, if you're confident, wait for the different units to call you. The process varies per DG and even per Unit. The decision is always with the head of unit, who relays his preferences. But sometimes the head of units delegates the decision to a particular person. Bottom line is this: you will likely receive a call from a unit which is specialized in a topic you might never heard of. Think quick on your feet and be enthusiastic. There is always overlap with what you did and what you want, and what they are doing and what they are looking for.