How to deal with model classes in iOS application

I am a newbie in iOS application development, but I am trying to learn how to deal with Cocoa in the best way.

I got stuck trying to understand how to keep and reference the model objects properly.

1. many say to write an app delegate property to hold the model and then reference it through the convenience methods for the app delegate singleton.
2. others say to "inject" in the view controller only the part of model which it needs (or its subviews needs), but I don't understand how to do this. Via a property? Via an initWithModel: method (and in this case, how can I say to IB to use that method?)
3. others again say that the model should be a singleton
4. and again, others say to use global variables (!)

Could you please give me some hint (and code samples)? I would like to learn the things in the proper manner, considering that soon I will be moving towards Core Data.