Trim it with a razor and straight edge, and then pull off the excess. Be sure to wash the paste off the wall right away. If you're lucky enough to have a handyman around, objects like the tank on a toilet can be removed. Then you merely have to cut a slit over to the pipe from the closest edge, and cut a hole. When the object can't be removed, slip the wallcovering down behind the fixture gently and smooth it down with a yardstick.
Ceilings Always hang the ceiling before the walls.
Make a guideline for the ceiling, like you do on the walls. Measure this distance out from both ends of the ceiling and mark with thumbtacks. Attach a chalked string and snap a plumb line. Have someone hold the rest of the strip for you as you position and smooth it down. If you're hanging the ceiling only, trim the excess with a sharp razor and broadkinife.