Hanging wallpaper in inside corners:
  
1.  Measure distance from the edge of the previous strip to the corner at the top, middle and bottom.

2.  Add 1/2" to the largest measurement and cut the next strip of wallpaper lengthwise to that size. (The portion you cut off will be used next.)

3.  Hang strip and smooth it into the corner allowing an extra 1/2" to overlap onto the adjoining wall. Trim off excess at ceiling and baseboard.

4.  Measure the width of your "cut-off" strip and snap a plumb line that same distance from the corner on the un-papered wall.

5.  Hang strip to the line overlapping the 1/2".