Access the ceiling surface from a stepladder at either end of the point where you wish to establish the ceiling line or measurement.
Measure from the nearest perpendicular room wall as a reference and mark the desired point for the ceiling line on the ceiling's surface.
Move to the opposite wall and repeat Step 2 to mark the other end of the ceiling line on the ceiling's surface.
Position a helper at the opposite end of the ceiling to access the first marked end of the ceiling line to hold the end of a measuring tape or chalk snap line on the mark.
Pull the tape measure so that it is held as tight as possible against the ceiling surface and read the dimension of the straight line measurement of the ceiling. To mark a line, hold the chalk line taught between the two end marks and snap a line by pulling slightly downward on the chalk line and releasing it to mark the line.