Already Painted Dywall
If the wall is already looking good and does not need much fixing up to do then the only thing you will really need to do is to wash them and then add some acrylic primer to the wall. Once this has dried then the wallpaper is ready to apply to the wall.
This is just as simple as before but without the wash, if there is no paint on the wall then the first thing you will want to add is alkyd primer, this is to make sure that you do not have any mould problems. After you have done this and it has dried then you can add the acrylic primer.
Older Plaster walls
Houses that were built in the mid 20th century still have some of the same plaster walling as when they were built, although this is sometimes a nice look for a house wallpaper can also add a noce touch. So the best thing to do in these cases is the same as Drywall, make sure you use the Alkyd primer, let it dry then the Acrylic primer.