As you have found with the FI course the skill test (Assessment of Competence in EASA parlance) is equally friendly and productive. The ground part is to assess that you are able to communicate adequately such that the student learns, just the same teaching in the aeroplane. It will be assumed that you have the minimum knowledge. A major required skill that is assessed is are you in control of the brief both long and short. The examiner may ask questions not relevant to the brief as a student will do in real life. It is expected that you do not allow such distractions but keep the brief on point. Remind the examiner, as you should a student, that the question will be dealt with at another time when appropriate: i.e. if it is a short brief do not be distracted into the long brief detail. A short brief is to prepare the student for what it is to be learnt during the flight that follows and what they should achieve.
Most important is not to guess or bluff which is the worst crime that any instructor can make. What you wrongly tell a student will stay with them for ever. If you do not know the answer then say so. Whatever you do, do not dig holes. Stay within the limits of your knowledge. Remember the level of training will be for an ab initio student to fly an aeroplane safely and not that of an aerodynamicist or a lawyer. You will be given notice with regard to the long brief a number of days before meeting. You should be able to produce the short brief in 15-20 minutes on the day and it should be no longer than 5-6 minutes in presentation.