Wallpaper is less forgiving than paint. Paint sinks into a surface and hides small sins; wallpaper sits on top of it and reveals them. A bump under paper stays a bump, and a seam that lands on an uneven patch will keep drawing your eye for as long as the paper is up.
So the wall comes first. Old covering off completely, adhesive residue washed rather than painted over, holes filled and sanded flush, and the whole surface primed with a product meant for wallcovering. That primer does two jobs: it gives the adhesive something consistent to grip, and it means the paper can later be removed without taking the drywall face with it.
Then layout. Where the first drop lands decides where every seam after it lands, and a pattern that is planned from the wrong edge can leave a sliver of mismatched repeat in the most visible corner of the room. Pattern repeat, drop direction and how the run meets doors and windows are worked out before anything is cut.
It is exacting work, which is exactly why it suits repeat spaces — hotel corridors, feature walls, guest rooms done to one specification. Get the wall right and the pattern planned, and the result holds up for years.