Good measurements are cheap; bad measurements are very expensive. It's extremely important to have accurate measurements of your wall, and to compensate for potentially sloped floors or ceilings.

Bleed is that extra material that we recommend adding to all four sides a gives you added play for variations in the wall height and width things being slightly out of plumb and square and give you little extra to cut off to get a crisp clean edge that matches right in the corner up at the ceiling are down at the baseboard We recommend  a minimum 1-inch of bleed on all four sides for safety, so a 90 inch tall by 120 inch wide wall you should order a minimum 92” x 122” wallpaper

Measure it’s important to measure your walls in more than one location. Take note if you have vaulted ceilings or if you’re covering the wall were stairs go down you need to be sure you measure the tallest and widest part of the wall double check your measurements and check in multiple locations. 

From Wallcoverings Association:

"The most important step in estimating wallcovering is accurate measurements. Use a yardstick or steel tape measure, never a cloth tape measure. Take measurements in feet rounding off to the next highest half foot or foot. Draw a room diagram showing doors, windows and ceiling height. If a wall is unusually broken up with a fireplace, built-in bookcases, etc., a diagram with detailed measurements will be beneficial in figuring square footage of wallcovering needed."

Wall measure oneWall measure two