Next, nail up the outside casing trim. These trim boards are cut slightly narrower than the plinth block and secured to reveal about 1/4" of the side jamb. Install the casing trim. Space the nails about 12" apart (Image 1). To match the top casing trim on the other doorway, a flat 1/4" by 1-1/2" piece of decorative trim is nailed on top of the two vertical casing trim pieces and just below the top casing trim.After the two side pieces and the thin decorative trim piece are in place, nail the top casing trim in place (Image 2).