Wallpapering over wallpaper is not the most recommended method for decorating your walls.
One or 2 layers might not be so bad if you prepare the wall properly but an more than that and you
could have a real mess on your hands the next time you want to change your walls. Removing 5 or 6
layers of paper is no easy task so you might as well bite the bullet and remove that 1 layer before you put more up.
If you insist, however, on wallpapering over wallpaper, you will need to prepare the surface properly.
Wallpaper will magnify any bump, hole, or crease that is underneath it so you will need to make the existing wallpaper
smooth as can be.
The first step is to sand down the seams of the existing wallpaper. Use a 150 grit sandpaper and make them as level
Remove any bubbles by slitting them open with a blade and re-glueing them down smooth, make sure you do not leave any
creases and remember to wipe off any excess glue. If there are creases, sand those down to be as smooth as possible
Finally, make sure you remove and / or sand any curling edges or loose pieces of paper.
Check around the entire perimeter and don't forget to look around window and door casing to make sure there are no loose areas.