When you come to a window or door, start by applying paste to the wall to the equivalent of 1.5–2.5 strip widths, as normal. Use a brush around the window or door frame.
Pull up the strip in the normal way, align the pattern against that of the previous strip and fix it in place at the top against the ceiling/cornice.
Feel and mark where the top corner of the window or door frame is with your finger and pull back the part of the wallpaper that is covering it. Make an incision from the top of the corner moving diagonally downwards towards the edge of the strip.
Now you can press the wallpaper down next to the upper edge of the window or door frame and cut the edge. Use the wallpapering tool to press down the wallpaper. Don’t forget to have a sharp blade in your craft knife!
Do exactly the same along the side of the window/door frame: press the wallpaper in place using the wallpapering tool and cut the edge. Repeat below the window frame (doors have only three edges to fit and cut around). Make the incision here from the bottom moving upwards, otherwise you may risk ripping the wallpaper.
So there you have it! Now that wasn’t so hard, was it? Just make sure to change your blade regularly, and don’t forget to wipe the surfaces with a damp sponge afterwards to get rid of any paste residue.