In order to exercse the privileges of a Instructor Certificate you must be qualified to act as PIC of the aircraft. In the case of a FNPTII that would involve either a MEP or SEP Class rating. If you are teaching the IR you would be required to hold a relevant IR and of course a vald CRI(ME) if its for Multi Engine. The alternative is to be quatified as an STI.
FCL.915
Applicants for the issue of or holders of an instructor certificate with privileges to conduct flight instruction in an aircraft shall:
(1) for licence training, hold at least the licence or, in the case of point FCL.900(c), the equivalent licence, for which flight instruction is to be given;
(2) for a rating training, hold the relevant rating or, in the case of point FCL.900(c), the equivalent rating, for which flight instruction is to be given;
(4) be entitled to act as PIC in the aircraft during such flight instruction.