The CFI-Instrument is a pretty straight forward check ride. You don't have to go through all the FOI stuff like you did for the initial and there are only a few required Area of Operations and Task to hit. Take a look at page 20 of the PTS. Yes, we are still using the PTS for CFI check rides.
A short word about the the PTS. I like to call the CFI PTS the teaching document. It tell us that we should be able to show instructional knowledge, demonstrate and simultaneously explain, and analyze and correct common errors. What the PTS does not tell us is to what standards we must perform to while flying. For example: Max 3/4 deflection of the Glidepath or Course guidance or +/- 100 feet of the assigned altitude. For those standards we must look to the Instrument ACS. I call the ACS the performance document for the CFI-I check ride. It tells us to what standards we must adhere to while simultaneously demonstrating and explaining any Tasks.
One of the biggest issues I see during the oral is Area Of Operations III, Task B; Cross-Country Flight Planning. Its a required Task and applicants flounder with it. I don't know why. Here's what I suggest. Take the CFII PTS and the Instrument ACS (Area of Operation I, Task C; Cross-Country Flight Planning) and place them side by side. Now make a lesson plan from the two documents. It doesn't need to be a text book lesson plan but something as simple as a checklist for you to go through that covers all the material in a logical manner.
Another weakness found during the Oral is the lack of knowledge in Alternate Minimums, IAP, DME Arcs and GPS. Since we may not be near a DME arc to shoot one, we have to ask how to you would teach it to a student (Note, the DME Arc is required. Reference CFII PTS page 1-20, see Note). If you have a G1000 equipped aircraft, how would you teach the Arc using that equipment. I can think of 2 legal methods. I'd use the easiest and least confusing.
A little bit about the flight portion. If there is going to be problems, they are usually the same we encounter with regular Instrument check rides, like busting altitudes, pegging the glide path or course needles. Usually these errors can be prevent by flying a few more hours under the hood from the right seat. By the way, did you know that you do not have to fly the entire check ride under the hood? Check out the comments in bold at the top of page 7 of the PTS. Some DPEs may require you to fly the whole thing under the hood but the minimum they are required to see is listed there.