How do you astral travel in to people's bodies?

DejaVu is absolutely correct. Astral Projection aka remote viewing is only possible if you have the correct genetic make up and brain chemistry/structure for it. It should come naturally. I don't know if it's possible to actually go into another individuals body as that is their temple but demons can certainly posess and dwell in people, so maybe ask them? Honestly I don't know why you would want to be inside another persons body, unless you've watched the movie Being John Malkovich and want to experience something like that.