When I had to install some crown molding I began by using a loop of string tacked to the wall near the ceiling to hold up the free end of the molding. The problem was, I kept scratching the freshly painted wall using this method. I had better success with the setup shown in the drawing below. I used a bar clamp to make a mast that extended beyond the top of a stepladder, and I clamped a handscrew to it to make a horizontal shelf to support the molding. Standing on another ladder, I could easily tilt the workpiece into position and snug its coped end against the adjoining molding while the free end of the molding rested on the handscrew shelf.
John Ketcham, Rockford, IL