Place a ladder or stool against the wall to give easy access to the ceiling.
Stand on the ladder and hold an angle measure tool against the wall. Position the tool so the hinged arm can be pulled out.
Pull out the hinged arm and swing it up until it touches the ceiling. Make sure the angle measure tool is pressed as tightly as possible against the wall and the ceiling.
Read the angle measurement on the hinged arm and write it down on a sheet of paper. Move down the wall and check the angle measurement every 5 feet, as there may be imperfections in the slope. Write each measurement down if it differs from the original.
Measure the width of the ceiling -- from one wall to the other -- with a tape measure and write the measurement down on a sheet of paper.
Measure the distance from the lowest part of the ceiling to the highest point of the ceiling with a tape measure. Measure against the wall to ensure accurate results. Use a laser tape measure for this measurement, if possible, as it may be tricky to use a traditional tape measure for tall or long ceilings.