Newly plastered walls and ceilings need to be sealed. For a full explanation as to why, please see our painting new plaster project.
To seal a surface before you hang wall paper the wall also needs to be totally dry. Information regarding this is also contained in the above project.
Sealing a surface for papering can be done using a solution refered to as a "Size" or "Sizing Solution". If you go to your local diy shed or builders merchant and say you wish to seal a newly plastered wall for wallpapering and require a "size" or "sizing solution" then they should be able to point you in the right direction in terms of the products you will need.
Before you purchase make sure you read the manufacturers guidlines in detail and ensure that it is suitable for your wall surface.
Many of these types of sealer come in a gelatinous solution which is mixed with water and applied straight onto the wall and allowed to dry. Many products of his type are also used in glazing paper and for stiffening textiles.
Most can also be papered over while still wet but, with the addition of wallpapering paste it sometimes makes the wall too slippery to hang paper properly.
A dilution of the wallpaper paste you are going to use can also be used as a form of size to seal the wall. Most pastes will indicate quantities to use for a sizing solution mix on the packet but a rough guide is to use 25% more water in a sizing solution. In essence a size or sizing solution is a little similar to a "mist coat" when painting.
When you intend to apply a vinyl paper to the surface, it is best to prepare the walls by sizing with a dilution of a paste with a fungicide in it. Vinyl papers are air proof and as a result any dampness at all that is trapped behind the paper will not be allowed to evaporate out. This could turn to mould.
Once you have purchased or mixed your "size" all you have to do then is apply to the surface using a large emulsion brush ensuring that you get a nice even speread and covering across the whole surface.
Once applied leave to dry and you're done!