The most important part of installing wallpaper is the prep work. The walls must be primed with a specific primer called sizing. Once that is done you need to put a level on the wall and draw a level vertical line. You will line up the first piece of paper with this line. Then you need to brush or roll wallpaper paste on to the backside of the paper. One you touch the paper to the wall, you get it in place and get all the air bubbles out. The paste will squish out on the sides, which is ok, just wipe with a wet sponge. Line up the pattern with the next piece and repeat.