Corners of walls are rarely perfectly square. When you reach a corner with your wallpapering you should cut the strip of wallpaper the width required to reach the corner plus 1″ (25mm).
This will allow the wallpaper to just go around the corner by 1″ (25mm).
If this 1″ (25mm) section of wallpaper won’t stick flat on the wall, then simply make cuts in it so it can overlap itself and stick flat. See fig 1.
Using your tape measure, measure out from the corner the width of your soaked wallpaper plus 1/2″ (12mm).
Mark this measurement on the wall in pencil, then using your plumb line hanging in line with the pencil mark, mark a vertical guide line down the wall.
Hang the next strip of wallpaper from the corner covering any cuts you had to make in the previous strip whilst making sure the other edge of the strip of wallpaper is inline with your plumb line so when you continue wallpapering each strip will be hanging perfectly vertical. See fig 2.
Using your wallpaper brush, remove any trapped air bubbles and trim off the excess wallpaper from the top and bottom of the strip of wallpaper (see post on pasting and hanging wallpaper).