Set one plinth block upright and flat against the wall face at the bottom of the left door jamb. Adjust the block to expose 1/8 inch of the jamb. Nail the plinth block to the wall and the jamb with finishing nails. Repeat on the right side of the doorway.
Measure 1/8 inch up from the lower edge of the top of the door jamb at the left and right corners of the doorway, and mark them with a pencil.
Measure from the top of the left plinth block to the 1/8-inch mark at the top of the door jamb. Measure along a piece of fluted trim molding that distance and mark it with a pencil. Repeat to measure and mark molding for the right side.
Set the base and blade of a miter saw at 0 degrees for a straight cut.
Lay one piece of molding flat on the saw's platform. Align the pencil mark with the center of the platform. Lower the saw blade without engaging it until the blade touches the pencil mark. Hold the molding against the platform fence, engage the blade, and lower the saw blade to cut the trim at the mark. Repeat to cut the other piece of molding.
Set the left molding vertically against the wall with the bottom edge against the top of the left plinth block. Adjust the molding to expose 1/8 inch of the jamb. Nail the molding to the wall and jamb with finish nails. Repeat the process on the right side of the doorway.
Set one rosette on top of the left molding and center it. Nail the rosette to the wall, and repeat on the right side of the doorway.
Measure between the inner edges of each rosette. Measure a length of fluted trim molding to that measurement, mark it and cut it with a miter saw.
Set the molding into place between the two rosettes. Adjust it up or down to reveal 1/8 inch of the jamb, and nail it into place.