A plinth block can be as simple as a square or rectangular piece of wood that's at least as thick as the molding that will run into it. However, if you're going for a more decorative look, you might want to add some molding.

Measure out and cut miters into some simple half-round molding, so that you'll be able to form a raised square inset from the edge of the block. Glue the molding in place.

Remember to make a plinth block for each corner of the door.

When your plinth blocks are complete, paint them and then nail them in place.