#1 rule in instrument flight:
Never get airborne to fly any procedures unless you have practiced them in a sim on the ground beforehand.
Otherwise you are wasting your time and money. If you don't know what to do in your armchair at home, learning it in the air is pointless.
I agree that it is better to learn in a sim first, but as an IMC instructor I wouldn't want you teaching yourself in a flight sim before hand.
I have taught many people how to fly instrument approaches without the benefit of a simulator, it isn't impossible, though of course using a proper sim is very preferable. If you have a decent fixed base sim available, then you would be an utter fool not to use it.
In 15 hours as long as the student isn't a total donkey, you have plenty of time to teach ILS's, NDB, SRA etc. The important thing is that at the beginning you get their basic instument flying up to speed before you go anywhere near an approach. Once people start to "get the picture" then you can move on.
Once you have been taught the principles, then practice them.