My CFII was probably the easiest checkride of my aviation career. It didn't hurt that the DPE who did it for me was and is a close personal friend, but he didn't cut me any slack on that.
If this is an add-on to an existing instructor rating, FOI material is not required per the PTS, since you've already been tested on it in the past. That being said, you should know it as a CFI so it is always fair game, and a good examiner will be looking for you to apply FOI principles during the ground and flight portions of the ride, even if you're not questioned specifically on them.
My ride was straightforward. The DPE advised me well in advance that I was expected to teach holding patterns and magnetic compass errors during my oral. On the flight, I flew a GPS approach, circled to a touch-and-go, then shot an ILS to a full stop. I did most of my own flying ... the DPE took the airplane so I could brief approaches, etc. I know of some DPEs, though, that will do most of the flying and leave the applicant to teach ... just depends on the particular taste of your examiner.
I do recommend that, if possible, you talk to some other people who have done CFIIs with this examiner to get feedback on their "typical" ride. That can be very helpful in preparation.
Good luck!
R