There are plenty of people who try to decorate their homes themselves and instantly regret it. Paper peeling off the walls, air-bubbles and gaps in seams are commonplace. It is even harder than it looks to gain that smooth and seamless finish especially without preparing the walls correctly first.
I use a specialist wall sealer to size the walls and a high quality ready mixed paste as I want the paper to stick to the wall for as long as you want it there and only to be stripped off only if or when you want a change and not because its peeling of and looking shabby hence forcing you to.
On very bad walls that have a good chance of seams opening ie over walls that have been painted then wallpapered and the paint is very flaky due to being stripped I often line the walls first horizontally so then as the wall paper is hung vertically it ensure no joins can open as we have completely crossed the joins so basically this creates a weave pattern albeit on top of the lining paper of course.
I also never overlap wallpaper as this looks awful and generally peels in time. I create my own joins by a technique of splicing the papers. This not only perfectly continues the pattern in a paper but looks fantastic around windows – especially if there are only Blinds and no curtains at all.