Of course, the student must have a mental 3D picture of the approach, and her position in it, created in her mind. However, in terms of initially intercepting, regaining or maintaining a particular QDM, I see, and teach, this as a pure DI/RBI or a pure RMI exercise - no mental picture is necessary, in my opinion.
I teach, put in its most basic terms, if the RBI/RMI needle head is left of where you want it, turn left, far enough to put the needle head onto the right of the instrument, so that it will fall right, back to where you want it, and vice versa if the needle head is right of where you want it, of course. Now, we know to what degree you do this depends on several factors, ground speed, how far you are off the desired QDM, how close you are to the beacon etc., but essentially that's it, I think. For QDRs, I teach the same sort of approach (ha ha - excuse the pun) but to pull the tail of the needle. I hope this helps a bit.
Regards, (waiting to get shot out of the sky) GT.