I've come to this late and you obviously didn't need my bit of advice which is to size(sic?) the walls. Basically, "paint" them with paste before you try to paper them and let it dry. It seals the surface and stops the paste from the paper just soaking into the wall and preventing you from being able to move the paper when it's up. And if you are buying patterned paper, make sure each roll has the same batch number, not just pattern number otherwise you may get differences of shade between rolls.
I've never been able to use a plumb-bob. I could never get a straight line from one. Using a spirit level you can draw a line - does the same.
