Starting papering by the main window
A little planning will make all difference.
It is most common to start next to the main window in the room and work away from it. This prevents shadows being cast by any slight overlaps at the joins.
Allowing for the pattern
If, however, you have a main feature like a fireplace and the paper has an obvious pattern, it is better to start there. The pattern needs to look balanced, so hanging the paper symmetrically about this feature will enhance the work. You will have a choice of hanging one piece of paper down the centre line, or hanging one piece either side of the centre. When deciding which method to adopt, look at the effect of this on the pattern itself, as well as the other drops of paper around the room. You may find that choosing one or the other gives you a less awkward cut at some point.
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.