For DEC there is only a huge questionnaire (about 300 questions) asking you questions about your personality and whether you strongly agree through to strongly disagree with the statements. You can't 'prepare' for that just go and do it, that's kind of the point. Don't think too hard about what they want you to say, as there's so many questions you'll end up sounding inconsistent trying to remember what you've put for earlier similar questions. This is followed by a Skype interview with a psychologist. I would recommend doing both of these before your assessment day, as it's good to get feedback etc .