Do it in a twin and then you'll have a leg up on the MEI. Get a few key instrument manuals for reference, I assume you already have the Jeppesen manual, supplement that with the Peter Dogan book and the Bill Kershner books. As far as the written goes, you need to use a standard test prep book and/or software such as Gleim and start taking practice exams. It's the same written as used for adding an instrument rating, the exact same one. As for the practical prep, fly lots of approaches and do the airwork just like you were an instrument student but sit in the right seat and practice telling a pretend student how to do everything. Have them play dumb and make few intentional mistakes, also. I recommend using US Air Force basic attitude-instrument method rather than the FAA primary secondary instrument teaching method, the latter is really confusing.