How do I install crown molding if the walls and ceiling are concrete? Any suggestions!

Rip a 2x2 in two on a 45 degree angle. Attach this triangle shaped material in the corner of the wall and ceiling with screws and anchors. Then nail through the center of the crown into the corner material using a finish nailer. Nailing through center is not something you would normally do because of the tendency crack the molding. Using a nail gun over comes this problem.