Once you get a valid score in CAT and you get a call from NITIE you can start preparing for the GD, PI and WAT process. If you are pretty confident about your score and which college you want to get admitted you can start prior to the result.
Now coming to your question about how to prepare for interview for PGDISEM:
If you have any doubt, connect with your mentor assigned to assist you. I hope this answer will help you and any future aspirants of NITIE. Just give your best and be confident. Wear that smile of confidence and just hold your thoughts. You will definitely get admitted to NITIE. Good luck.