Initially, we talk about it, then dim the screen and work on the Four Fundamentals (after the can do the 4F's full panel). After that at each stage (basic radio nav orientation/interception/tracking, holding, approaches, etc), once they can do it full panel, we do it with the PFD down. Once they can do it, I move to the "surprise" mode, where I generally kill the screen while they aren't looking, like when their eyes are closed for unusual attitude set-up. They open their eyes and see black glass, which often startles them. After they recover, I send them to fly an instrument approach that way. If I'm in a plane where I can pull the c/b and have them look up to red X's, I'll do tht at that stage, too, or even just pull the c/b while they're doing something under full panel, although it's usually hard to pull the c/b surreptitiously.