I have learned the hard way.
Not all wall paper needs to be removed or should be removed.
All new sheet rocked walls need paint on them before applying wallpaper.
Many new homes I have worked on do not have paint on the substrate (Sheet rock).
Later on down the road someone will want to change the wallpaper or paint the walls. Removing the wallpaper turns into major repairs. If only the new substrate was painted removing the paper would be less work and less cost to the home owner.
1. If the drywall was not painted and you are going paint the best way I have found is to apply oil base kills directly over the wallpaper repair the wallpaper joints and go from there.
2. If the drywall was not painted and you are going to paper, apply sizing and paper right over the top. You must inspect the joints and make sure they are tight and all paper is tight. Nine times out of ten the paper is glued to the wall. Do not put joints on top of joints offset the joints.
When wallpaper is applied directly to unpainted drywall the glue absorbs right in to the paper and the new wallpaper becomes part of that wall.
If there is a coat of paint it can be any type of paint removing the paper at a later date will be much easier and less cost to the home owner.
Sizing acts like extra glue and lets the new wallpaper slide when matching up the print at the joints.
I have hung thousand of rolls of paper. I no longer offer this service.