First of all, you have to remove all items that are on the wall. Old wallpaper, hooks, nails, covers of light switches and power outlets ...
Ideally use sanding paper and smoothen the wall. Then use sugar soap to give it a quick wash. If there is any loose paint, scrape it off and use a thin filler. Once the filler is completely dry, use a sealer/undercoat to paint the wall like this one from Zinsser. The advantage of the sealer undercoat is that it created an evenly absorbing surface and dries within little time. Now your wall is ready for the wallpaper installation.
This wall was not yet ready for wallpapering:
The problem with this wall creates many problems:
A) The uneven colour can show through the wallpaper and after the installation you can see where it was not pained.
B) The filler absorbs the wallpaper paste different than the Gyprock panel and the painted area. You could end up with parts of the wallpaper that is not sticking or wrinkled as the paste has dried too quickly.
C) In case there is no undercoat used, it can happen that the wallpaper pulls the single paint layer off and sticks to the backside of the paper.