Applying filler wallpapered wall
Remove any remaining wallpaper.
Apply primer/protective primer to the substrate as required.
Apply filler to uneven surfaces and joints.
Repeat application of filler until the subsurface is smooth.
Sand and dust down the filled surfaces.
If the wallpaper is going to be painted over, the whole wall needs to be fine smoothed, so that structural differences are evened out. Then sand. Fine smoothing is not needed for wallpapering.