Step 1
Line up your square decorative trim block so that the inside corner of the block is three-sixteenths inch from the outside corner of the door jamb. Hold a level on the block to keep it straight. Nail the block to the studs using at least three nails. Repeat for the second block at the other end of the top of the door jamb.
Step 2
Measure the distance between the blocks. Cut trim molding to this length. Line up the bottom of the molding with the bottom of the two blocks. Nail the molding to the underlying studs using finish nails. Nail every eight inches with a nail gun.
Step 3
Measure the distance from the bottom of the block to the floor. Cut trim molding to this length. Line up the molding with the inside edge of the block. Keep the molding three-sixteenths inch from the inside edge of the jamb. Nail every eight inches with a nail gun. Repeat for the last side.
Step 4
Fill the nail holes with wood fill. Press the fill into the hole with your finger and wipe off the excess. Caulk your seams and joints with a caulk gun. Apply a narrow bead. Wipe off the excess with a wet-tipped rag.