To install floating crown molding, use a hidden strip of wood to screw the bottom of the crown molding into, securing it to the wall and making it appear as if it is floating.
Purchase already cut crown molding to your size specifications or cut your own. Measure the location you wish to install the crown molding and mark it with a pencil. Next, locate a small piece of wood approximately ¾” thick and 1¼”  high. It will need to fit in the void space caused by the angle of the crown molding.
Once level, screw the wood piece into the wall. These screws need to go at least 1” deep into the wall studs for maximum security. If the piece of crown molding is 6” deep, a ¾” thick wood strip should offer enough support. For a ledge that is deeper than 6”, use more substantial wall cleats to hold the crown molding in place.
Next, add your crown molding on top of the secured wood strip. This method can be used for simple crown molding or to make a floating crown molding ledge.