As a first step, you must remove any old paint or wallpaper.
Prepare your wall by filling in any cracks, small holes, and irregularities, as these would be visible once the wallpaper is installed.
Use a lamp to carefully inspect the wall and ensure it is smooth and even.
Wash the surface and make sure it is clean and smooth.
If the wall is absorbent (such as plaster walls, walls with old wallpaper, drywall, or similar), apply a coat of primer or diluted glue to limit glue absorption during installation.
If the wall has strong colors or color variations that may show through the wallpaper, apply a coat of white paint.
Begin installation only after the wall is completely dry.