Trimming paper around external corners
The easiest way to paper external corners (which are rarely square or vertical) is to take the paper around the corner by about half an inch and trim it to this point.
The next section of paper can then be hung vertically, overlapping this half inch and trimmed a fraction back from the corner.
The professional alternative, which avoids an overlap, is to run the first piece around the corner by a couple of inches. The second piece is then hung ovelapping this, taking care to match the pattern. The two pieces are then spliced using a straight edge and sharp knife. The edges are lifted and the excess from both pieces removed. The two edges are then smoothed down creating a perfect joint.
Trimming paper around recessed openings
Hang the length which overlaps the recess.
Cut a horizontal line a fraction above the head of the window, back to the top corner of the recess.
Cut a line a fraction below the top of the sill, back to the bottom corner of the recess.
Now smooth this middle section around the side of the opening. If the side of the recess is very ‘out of square’, treat it as an external angle as before.
Now, allowing for pattern matching and trimming, cut a piece to suit the recess at the top, as if it were a continuation of the length just hung.
Paste this piece and position it, carefully aligning the pattern.
Slide the front edge up under the main length.
Trim the two pieces carefully just above the head of the window, using a sharp blade and straight edge.
Remove the offcuts and smooth back down.
Continue papering above and below the recessed frame, treating the top as an external angle.
As you reach the far side, treat this exactly the same as for the first side of the recess.
Be sure to hold the paper a little away from the wall and line it up with the edge of the last pieces above and below the recess before pushing it back to the wall into position.