To make a crown molding ledge for pictures, you need to measure, cut, assemble, and hang your new shelf ledge; you are building narrow boxes to create the ledge.
This is a DIY project which requires attention to detail. Determine what size ledge you want, then gather your crown molding pieces. These can be unfinished solid wood crown molding to leave as is or paint later or choose already painted versions.
Next, you need to make precise cuts, creating the shelf top, box front, box sides, box bottom, and wall cleat. For a 36” shelf, build a base with slightly smaller measurements. Assemble these base pieces with pocket holes and wood glue. Allow glue to dry.
To complete the assembly, line up the top shelf and attach with more glue, then cap the bottom of your box the same way. For hanging, drill holes in the back edges of the shelf, approximately ½” from each edge. Depending on how much weight you plan to place on your ledge, design the best size wall cleat and screw it to a wall stud. Then, place your shelf on top of it and secure it in place.