An even wallpaper finish requires a clean, smooth wall. Scrape or sand any rough spots, and fill in depressions and cracks with spackling compound. After washing the walls, "size" them with a good wallpaper primer or sizing solution to keep the paste wet longer and the paper from shrinking as it dries.
Begin papering in the least visible corner. Use a plumb line to guide your placement of the first strip and all corner strips thereafter. Let the edge of each corner strip wrap around the corner, creating some overlap where corner strips meet. Trim the excess paper by using a razor knife to cut along the corner, from ceiling to floor, directly in the point formed where the walls come together. Peel away the trimming.
When papering around windows and doors, let the edge hang over the edge of the trim molding. Use scissors to cut into the paper from the loose edge into the corner where the trim meets the wall, then smooth the paper flat and tight into the corner. Trim away the excess paper with a razor knife.