How to install crown moldings in rooms
1. Decide whether you want to paint or varnish your crown molding before or after installing it. If you think you will have a difficult time reaching all areas of the molding after installing it, paint the pieces before hanging them. If you will have joints in the middle of your wall, painting after installation can help make them less prominent. You may want to apply a primer and possibly a first layer of coating first, then install the crown molding and paint another layer over it later.
2. Next, you need to create a template so you can mark the walls. If you create a corner as explained here, you can use it to ensure the angles of your cuts are correct. Use a miter saw to cut the molding so your pieces join at the proper angle, then join them using Loctite Power Grab All Purpose or Loctite Power Grab Ultimate and position them below the ceiling. Now you can mark where the edge of the molding will be and apply similar markings everywhere you want to hang the molding.
3. Now it is time to measure, cut, and install your molding and paint it when appropriate. Proceed as you did when creating the template, then apply a sufficient amount of Loctite Power Grab All Purpose or Loctite Power Grab Ultimate to the molding or wall. Press the molding into place, hold it there for a moment, and repeat the process until the room is done. Apply some mechanical fasteners to support the crown molding, then caulk to finish the seams, top, and bottom. Remove any excess and let everything dry.