Working Around Doors and Windows
When your layout comes up to the casing on a window or door, don't try to measure and cut a piece to fit exactly because you can actually set a piece in place and cut it to fit around the casing right on the wall.
Measure and cut a floor to ceiling strip with the extra 4-6 inches for trimming the top and bottom, do the necessary activation of the paste and booking, and then set the paper loosely in place on the wall right over the casing.
Make sure the one edge butts up against the edge of the previous strip so your seam ends up right.
While holding the piece in place on the wall, make a diagonal cut from the edge over the door or window up to the upper corner of the casing, to the point where the paper will actually meet the corner of the casing.
On a window, make a second diagonal cut down into the bottom corner of the casing. Then use the broadknife to press the paper down along the vertical casing to where the casing meets the wall and form a crease there, making sure the paper's still positioned properly at the seam.
Make a similar crease across the top of the casing and on windows a third crease along the bottom of the casing.
Then use the broadknife and your razor blade to cut away the excess at the creases you made and smooth the paper into position. (Of course, the more detailed your trim, the more detailed will be the cutting required to scribe it to the trim.)
Cut and paste any pieces required to carry over the door or over and under the window.
Then repeat the process described above to wrap the other side of the door or window with paper.
Hanging Inside Corners
If you try to run a full strip along one wall and continue past an inside corner onto the adjoining wall, it can easily come loose or go askew if the corner's not square. The best technique is to create a seam at each corner.
Measure from the last plumb line to the corner at the bottom, the middle and the top of the wall. Take the longest of the 3 measurements (which accounts for any problems with plumb or square at the corners) and add a half-inch for an overlap at the seam.
After activating and booking a piece, measure and cut it to that width, using a straight-edge to mark the cutting line for your scissors or to guide your razor knife as you cut.
Save the waste and hang the cut piece on the wall, making a tight seam with the previous piece and pressing the other edge into the corner.
Then measure the waste piece, measure out that distance from the corner onto the adjoining wall, draw a plumb line through that mark and hang the waste piece with one edge on the plumb line and the other edge on the overlapping piece in the corner.
Run a bead of the adhesive over the bottom piece, spread it over the covered area and then lay the overlapping piece over it.
Smooth out the paper, trim the top and the bottom and sponge off any excess adhesive.
NOTE: If you're using a vinyl wallcovering, be sure to get a tube of vinyl-to-vinyl adhesive for use whenever you overlap two pieces--like in the corners.
Hanging Outside Corners
On outside corners, it's easier to bond the paper firmly to the adjoining walls so it's acceptable to wrap a full strip around the corner.
Just snip the trim material at the top and bottom where you need to turn the corner to wrap around prior to the actual trimming.
And be sure to smooth both sides and the corner thoroughly to remove air pockets which could lead to the paper peeling off the wall.
Turning the corner like that will be difficult if the corner's out of plumb or square. In that case, treat it like an inside corner and create a seam.
But on an outside corner, it's better to make that seam about a half inch past the corner to prevent fraying. So leave an overlap of a full inch and add a half inch when you mark the next plumb line.
TIP: If you're ending a wallpaper job right at an outside corner, don't paper right to the corner. Cut the paper about 1/8-inch short of the corner to keep that edge from fraying or peeling off the wall.