It is not an over night thing, you don't just do it one time and have it take effect -- Nor do you actually "replace" reality with fantasy. You always know what really happened.
Also it is (imho) more of an emotional overlay than a mental overlay.
I can give an example, once again from my childhood.
The last month of school the local bully had harassed me to the point of pure terror. Over the summer I pictured making him look like an idiot so often that when the new year rolled around I had lost all fear of him. When he threatened me I pictured pushing him backwards into a pile of dog dung and laughed. He was the first to tell me he was going to "Wipe that smile off your face." and, "You'll smile out the other side of your mouth when I'm done." I've heard them often since then.
My lack of fear had an effect on him and instead of hitting me he spit on me and walked away. He never bothered me again, but I laughed when he spit on me and this had an unfortunate effect: Even my friends did not want to associate with me after that and teachers looked at me askance. For some reason the idea that someone could laugh when being spit on struck everyone as being a bit too far to the left of abnormality.
I still know my mother died, but now I can't think of it without thinking of the time I was crying and she held me tight to her telling me "everything is going to be okay." And I believed her because she said it.