It’s like reading a book. While you are reading, you allow yourself to get into the flow of the action, become deeply involved with the characters in the story, and often identify with what they're feeling and experiencing.
You don't usually say, "This isn't really happening: it's just words on a page and my imagination is filling in the details." Chances are, you become so immersed in the story that your other senses get involved, too.
Becoming deeply engrossed in a book, a movie, or TV show is a form of hypnosis, and if you can do that, then you already have the ability to do a Past Life Regression.
Please note that, like any skill, you may have to practice self-hypnosis or meditation before you are successful at a PLR, especially if you have a busy mind or are highly skeptical.