How to Hang Wallpaper in a Corner
Take your wallpaper to the corner and tack up each side of the paper on either side of the corner. Before working on the corner, ensure your pattern matches with previously hung wallpaper. Gently push the wallpaper into the corner while smoothing out the side pattern matched at the seam. Reposition the wallpaper as needed.
With the pattern match set, use a smoothing tool to firmly push the wallpaper into the inside corner. Then, place the smoothing tool against the wall and use a sharp snap-blade knife to cut the corner. Be sure to work from the ceiling to the floor, changing to a new blade if necessary. Once the corner is cut, trim the excess paper from the top and bottom of the wallpaper section nearest the seam.
On the new wall, reposition the wallpaper as needed to match the pattern. Many inside corners are not perfectly square, so take care to ensure the outside wallpaper seam is straight and plumb. Smooth out any bubbles and trim excess paper at the ceiling and floor. Any residual wallpaper paste can be washed away with a damp rag or sponge.