Cut a piece of wallpaper large enough to extend from the ceiling above the window to the window and into the recess as far as the window frame, allowing some extra for trimming. Hang this next to the previous strip of wallpaper and use the brush to smooth it into the window recess. Trim at the ceiling level and at the top of the window frame.

Use a pair of paper hanging scissors to make a horizontal cut in the wallpaper just above the head of the recess from the edge to the top corner of the recess. If there is a bottom corner into the recess (there probably will be for a window), make another cut in the wallpaper at the bottom of the recess just below the top of the sill.

Fix and smooth the paper onto the wall above the window recess overlapping the paper already there, carefully matching the pattern whilst doing so. Make a double cut through both layers of paper and then remove the layer already stuck to the wall. Fix the new strip in place above the recess and then fold it around on to the overhang.