If your walls are glossy or have a silk-like finish you need to lightly sand them to in order for the new wallpaper to adhere properly. Use a fine course grid of sandpaper.
If the wall surface has quite a bit of dust on them, seal the dusty surfaces with a coat of PVA glue solution which is five parts water to one part PVA. This allows the paper to stick and also allows you to move the paper easily.
If you run into a situation where you can’t remove a highly textured coating with a steam stripper, you will need to apply a stabilizing coat of diluted PVA which is five parts water to to one part PVA, then coat the surface thinly with plaster. Steam strippers are inexpensive to rent for a day and should be considered if the size of the job is large.
To Hide Imperfections:
For Stains: Use a commercial stain-blocking agent or apply an oil-based primer before applying the wall paper to prevent stains from bleeding through the new paper.
To remove protrusions:
Make sure any nails or screws have been removed from the walls or these will cause staining to the paper. If your unable to remove nails or screws, hammer them in as flat to the wall as possible. If needed, use plaster to even out the walls surface around the nail or screw.
To cover dark backgrounds:
Apply a coat of light latex paint to dark or patchy walls when applying a light-colored wallpaper. This will prevent the dark color from showing through.