Step 1 : Measure and layout
To determine how many rolls you need, measure the length of the area you intend to border. Divide that figure by the length of the roll to find the number of rolls without a waste factor. It's a good idea to add about 1/2 yard extra for every rolls to allow for matching, and add an additional 1/2 yard for every corner or miter, such as when you are bordering windows.
Even some newly built homes won't have walls and ceilings where everything is true (level, plumb, and square), but you can establish a horizontal guideline with a carpenter's level and measuring tape to act as a guide for hanging a wallpaper border. To establish a level line below the ceiling, use a measuring tape and a carpenter's level to establish guidelines on the walls around the room.
When you decide where you want to install the border, mark the location on one wall at a corner. Measure down from the ceiling or up from the floor, depending on which distance is shorter. Hold the level flat against the wall at the point you marked and adjust the level it until the bubble is centered. Then draw a light penciled line on the wall at that mark. Make your way around the room using the level in the same manner to create a line on all the walls of the room.