Booking refers to how the wallpaper should “rest” while the glue activates. Booking allows the paste to penetrate the wallpaper while keeping the paste from drying out before hanging it.
Once the glue has been activated on pre-pasted strips or spread on un-pasted ones, you should lay the strip pattern-side down on a flat surface. Gently fold the ends of the strip back towards the centre so the pasted sides are touching.
Do not allow the folds to crease or you may leave a permanent mark. Follow the manufacturers recommended time for booking before hanging the strip. You do not need to “book” the wallpaper when using the Paste-the-Wall method.