Measure the length of the wall for the first piece of molding. This measurement should be taken at two places, if your walls tend to be out of plumb to any significant degree. Take one measurement at the ceiling and a second one a few inches below that, to ensure that they are the same.
Determine if the molding will meet at an inside or outside corner, and compute the compound angles necessary to make the joint. The angles will be determined by the type of crown molding that you are using and the angle at which it sits against the wall.
Add a bead of silicone caulking to the length of the molding, before nailing it to the wall. This will help to keep the molding from pulling away from the wall due to the weight of the wood.
Nail the molding to the wall and ceiling using the nail gun. The pneumatic nailer minimizes damage to plaster walls as it shoots the nail into the molding and wall, as compared to the pounding action of the hammer and nail.
Repeat Steps 1 through 4 for the remaining pieces of molding.