Go as far as you can without using a board. Then use a board to finish the line. Preferably the book you use to practice this isn't terribly difficult with tons of analysis. I think endgame books are easier, maybe start there.
I can go over lines without a board, sure. But if it's difficult analysis (especially with multiple lines) then I always get more out of it by setting up a board. Just because you can superficially visualize a new position, doesn't mean you can see everything that's going on, or the main point the author wants to illustrate.
I've never been able to understand how to comfortably understand a line of moves by looking at chess notations. I don't mean that I don't understand the notation. Algebraic notation (or the other types) is very easy, of course. What I mean is that, if I'm looking over an analysis or a game and the book goes into a bunch of lines, I have absolutely no way to understand the point without taking out a physical board and (painstakingly) go over them move by move. And, when a new line is introduced, to set up the pieces again and to go over the new line.
A friend of mine (who is about 1800 to 2000) says that he can go over the lines in his head. What would anyone suggest for a person of my level (about 1200 to 1400) who cannot do that and who doesn't always have the time or resources to set up pieces on a board to get a meaning.