Wallpapering Around Corners and Windows
Sooner or later you will run into a corner. If you are only doing one wall, then trim the side of the paper. If you plan to go on, you have a bit more work to do. As walls are seldom exactly square you do not want to simply wrap the paper around the corner, it will likely cause your next piece not to be precisely vertical. To deal with this on an inside corner, measure the distance from the edge of your paper at the top to the corner of the wall. Do the same for the bottom and the middle. There will likely be a variation. Add 1/2 inch to the largest of these numbers and make a mark at the top of your paper and at the bottom with the longer of the two lengths (both top and bottom will be the same - the width of the paper will be uniform). Cut a straight line connecting the top and the bottom marks. Now when you paste this piece of paper to the wall it should wrap around the corner by 1/2 inch at the narrowest point and more everywhere else. In other words, there will likely be a slant to reveal how far off square your wall actually is. You should have a strip of paper remaining (the other half of the piece you cut). Now measure the width of the remaining piece of wall paper. Measure out this distance from the corner and drop a new plumb line. Now put the piece in place using the new line as a guide. The match will not be perfect, but this will not be obvious in most patterns. The new strip will overlap the previous one. For vinyl paper, you will need to trim the second sheet where the first and second sheets butt together. This can be done by a process called double cutting.
Wallpaper can be hung around an exterior corner. Be sure to put a slice in the excess material at the top and the bottom from the corner so that the piece wraps around without wrinkling. Now use your plumb line or level to make certain your lines are still straight up and down. If not, find the point where the plumb line touches the edge of the paper at one point and covers the paper at every other point. Snap a line. You may also want to snap a line at a distance equal to the width of the roll of wallpaper. Use this as a guide, hang the next piece. Trim the overlap using the double cutting method described above.
Remember to work around the room in an organized manner. Do not skip any areas with the thought that you will come back to them later - this will undoubtedly cause trouble matching later. When dealing with windows and other obstructions simply leave about 2 inches of extra wallpaper at the proscribed openings. When laying on the paper, make a slit at the corners with you trimming knife. Press the paper against the side of the trim firmly with a putty knife or your trim guide and then cut away the excess material. For recessed windows and openings you can simply run your trimming knife along the top and bottom of the opening (not the sides). Then wrap the sides around trimming where desired. Then find scrap pieces for the areas that are horizontal. Conversely you may simply want to stop at the face of the opening.