Actually, I don't know how this is done.
My time slipping tends to be intuitive. I think of a person, place, or event in history and sort of will myself there. Sometimes I just fling myself into the past, or the future.
I'm actually just back from amcient Rome. t was a smelly place really and I couldn't bring myself to use their communal yoilets, so I rushed back for a dump. It is harder to slip through time when you are desperate for a dump. It sort of mucks up your concentration.