Few walls are truly square or perfectly vertical. To overcome this, so avoiding your pattern going askew, always mark a vertical pencil line against a plumb line or long spirit level (you can make your own plumb line with a weight attached to a thin string). Allow the plumb to swing freely until it is at rest before putting your pencil mark down the wall behind the string.
You normally start hanging in the least visible corner, often behind a door. The plumb line should be about 25mm (1") less than the width of the wallcovering away from the starting point (see fig. 7).
Don't hang on the line, but just leave it showing by about 5mm (1/4") and hang parallel with it. Next, smooth down and brush back the length away from the line and into the corner going round the corner by about 25mm. See also Dealing with corners.
