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Blemishes will show up more clearly through wallpaper than through paint. To check for blemishes, direct the beam of a flashlight along the wall. Blemishes such as paint spatters will stand out clearly. Smooth such blemishes with sandpaper. Feel the wall for depressions and fill them in with drywall compound.
Paint the top corners of the wall with ceiling paint. This will eliminate stark differences between the underlying wall and the wallpaper. Apply sizing to the walls so that the wallpaper will be easier to install.
Prime the walls with a high-quality oil-based primer. Primer is especially necessary for new walls because untreated drywall will quickly absorb moisture from the wallpaper adhesive.
Examine the design of the wallpaper and determine where you want the pattern to meet the ceiling. Measure out a length of wallpaper approximately 6" longer than the wall. Make a small mark just above the point where you want the wallpaper to reach the ceiling or crown molding. Use the first sheet as a template for cutting the next ones. To account for inside corners, add 1/2" to provide the extra paper that will wrap into the corner (this is called a lap seam).
Fill the wallpaper tray with lukewarm water. Roll up each sheet of wallpaper paste-side out. This will help the water to coat the paper evenly. Soak the paper in the wallpaper tray for about 15 seconds, making sure all the air bubbles have escaped. Slowly pull the wallpaper out of the tray, making sure the water has covered the paper, and lay the paper face down on the table.
"Book" the wallpaper by folding both ends toward the middle, paste side in; take care not to crease the edges. Booking activates the paste, helps to distribute it more evenly, and allows you to carry the wallpaper easily. To save time, book several pieces of wallpaper and place them in a plastic bag to keep in the room you're papering.
From the least conspicuous corner of the room, mark the wall at a point that is 1/4" less than the width of the wallpaper. Draw a plumb line down the length of the wall from the mark.
Unbook the wallpaper, and press the top portion to the edge of the ceiling. Make sure your pattern is lined up properly with the ceiling and that the edge of the paper is flush with the plumb line. Don't move the wallpaper around any more than necessary -- you might stretch it and cause the seam and pattern to be out of alignment.
The wallpaper may tend to wrinkle near the top corner. If this happens, cut a slit in the paper, starting at the edge of the ceiling and cutting outward. Use a sharp utility knife or a pair of scissors.
Use a broad knife or a wallpaper brush to smooth out the wallpaper and press it into corners. Start at the top of the wallpaper, and work your way down, using vertical strokes whenever possible (horizontal strokes can stretch the paper). Work from the inside area toward the edges to remove wrinkles and bubbles. Wipe up any excess glue with the wallpaper sponge.
Hang the second piece, starting at the top. Line up the seams as closely as possible, and try to match the patterns exactly. Smooth the second piece as in the previous step. Continue with each piece in the same manner.
Trim off the wallpaper at the top and bottom. You'll need a wide straightedge to press the paper as tightly into the corner as possible (Image 1). Use a sharp utility knife on the underside of the straightedge to trim the paper (Image 2). When you reach a corner, make an angled relief cut to help you fit the paper.