The ruler

If you use ordinary wallpaper of medium density, then you need to glue them over, and the edges respectively cut off only on one side. No need to break off the edge, it is better to use ordinary scissors to remove it. If you have heavy and dense wallpapers, and you need to glue them back, then you will need to get rid of the edge from both sides. Here you cut the edge under the ruler using a knife designed for cutting paper.