Take Michael Raduga's "easiest" method at the top of this page, to the first cycling technique of "Observing images". That's basically it, but without trying to separate first, or even having to sleep.
I've done this before sleep, immediately after waking from sleep, after a break from sleep (1/2 hour or so), and during meditation. Observing the hypnogogic/hypnopompic imagery until one image appears that is super-realistic. Usually this pulls me straight into the scene without having to separate first. Of course it requires that the body is very relaxed, and the mind is also relaxed but alert. The experience of suddenly appearing fully and physically in a scene that was first just an image is astounding. Sometimes I don't get full immersion, as if my mind is there as an observer while I'm still aware of my body in my bed - but in these experiences I can still interact with the scene, move around within it as a floating eye...
Maybe it's because I'm a strong visualizer that it works so well for me, but it's my favorite method, and even when I don't have full immersion, I still experience a phase state.