Door frame facings are typically framed with a picture-frame type moulding known as door casing. In most modern designs, door casing is installed with 45-degree mitres at the two top corners for a smooth, continuous line. Some styles call for a header or capitol that intersects the side vertical trim at the same height as the top of the door frame. Another recent style is to use corner blocks at the tops of the vertical side pieces to create decorative corners leading into the horizontal header. The styles have different cutting needs.
Measure from the floor up the side jamb to a point 1/4 in. above the bottom face of the horizontal jamb header. Cut the two vertical pieces with a 90-degree bottom and a 45-degree top mitre, one right and one left. Cut the mitre so that the shorter inside corner is the same as the distance you just measured.
Nail the two vertical pieces in place along the face of the door jamb, spaced 1/4 in. to the outside of the inside edge of the jamb. Hammer in a finish nail every 10 to 12 in.
Measure between the top outside corners of the installed vertical casing and cut a piece to fit between, with 45-degree mitres at each end, one right and one left. Position it snugly between the side casing and nail it to the header with finish nails.
Cut two vertical pieces with 90-degree cuts at each end so that they reach from the floor to 1/4 in. above the bottom face of the horizontal jamb header.
Nail the side casing in place as for mitred casing. Add one nail 2 to 3 in. out from the inside edge, into the framing stud, every 16 to 18 in. for wide trim.
Measure from outside to outside of the installed vertical casing at the top corner. Cut your header to length 1/2 in. wider than this dimension. Nail it in place, centred with its bottom edge resting against the tops of the vertical casing.
Cut and install your vertical side casing as outlined in the section on capitol casing.
Nail one corner block in place at the top of each capitol. If the blocks are the same width as your casing, make the outside edges flush with the casing. If they are wider, make the inside corner of the block flush with the inside top corner of the casing at each side. Rest the bottom edge of the block on top of the vertical casing. Place one finish nail in each corner of the block.
Measure between the two corner blocks and cut the top horizontal casing to fit. Nail it in place as for the side vertical casing.