Make sure you paste it properly - not too thick and well spread out over all of the paper missing not even a fingernail sized hole. Paste down the centre and draw out the paste to the edges.
Fold top and bottom towards the centre and give it a few minutes for the moisture to soak into the backing paper - you could even be pasting the next piece while you wait.
Get piece number one up on the wall and line it up where you want it at the top and on the matching side and smooth it down firmly with the palm of your hand but not so firm as to crease or rip it. When you've done about 18" or so take a clean rag (old teatowel) and firmly smooth the paper down the centre and out towards the edges moving trapped air and excess glue to the edges. Repeat for the next 18" or so gradually working down the wall. Trim top and bottom using the back of your scissors to make a line and wipe the paper back to the ceiling and skirting. The paper should be pretty flat with no bubbles but you may need to take a wet sponge and dry cloth to wipe away any excess paste that has squeezed out from either side. If you don't do it now you end up with shiny marks on the edges when it dries.
The paper may well continue to absorb moisture and a few small bubbles may appear over the next few hours but ignore them as they will fade to nothing as the moisture dries out - honestly.
The trick is not to be in too much of a rush to get the paper on the wall after you've first pasted it - be patient.
Peccavi
December 2008