Decide how large you would like the built-up molding to be.
Pick out and purchase several moldings that can be used during the build-up process. Be sure to get them in various widths and detail.
Lay out the moldings on top of one another.
This will give you a good idea of what the final layout will look like when it is installed on the wall.
Install the first piece of molding.
This molding should be the base piece that sits against the wall. Measure the wall and write the measurement down on the molding. Cut it to the correct size with a miter saw and nail in place with a pneumatic nail gun.
Install the rest of the moldings on top of the first piece.
Cut and nail each piece of molding into place with a pneumatic nail gun. As you stack each piece, you will build-up the molding.
Apply caulk to the gaps between the pieces of molding and between the molding and the wall.
Smooth the caulk with your finger. Let the caulk dry. Paint as desired.