Obtain a wallpaper scoring tool. These can be picked up at any hardware store, borrowed from a friend or found in your tool box. Some popular brands are PaperTiger and Simple Strip.
Press the tool firmly against the top of the wall and move it downward, maintaining pressure. If the tool doesn't penetrate the wallpaper, redo the action and press harder.
Repeat step 2 for each section of the wall, until all of the wallpaper has been scored.