Plumb line for paper hanging
Once you decide where to start, mark a vertical line down the wall. You can do this either with a spirit level or with a plumb line.
To use a plumb line, mark your starting position on the wall close to the ceiling. Fix the end of the plumb line over this mark with masking tape. Allow the weight to hang freely and steady it.
Now make several pencil marks alongside the string. If you like, join these marks with a ruler to give a complete vertical line from top to bottom.