Measure the center line of each of the door hinges, then measure vertically from the floor of the doorway opening on the hinging side and mark the hinge locations on the trimmer stud.
Check the door’s rough opening using a level to verify the walls and trimmer studs are plumb on both sides of the door opening. If either trimmer is not plumb, move it accordingly driving the tongue of a pry bar between the trimmer stud and king stud to adjust the trimmer, then slip wood shims into the gap until tight and secure the shims in place with finish nails. If the hinge side trimmer has been adjusted, be sure to insert shims at the hinge locations marked in Step 1 to provide secure backing.
Place a level on the floor in the doorway to check that the floor is level. If the hinge-side of the doorway is lower than the door’s latch side, raise the level using wood shims until the bubble in the level window is centered and tack the shims to the floor as a support the jamb on that side. In case the latching side of the door opening is lower, shims will not be needed for the door installation.
Separate the spit jamb sections and insert the door side of the jamb assembly into the rough opening, making certain the rest the hinge side of the jamb tightly against the trimmer stud, as well as directly on any leveling shims installed in Step 2.
Secure the door side jamb section in place by tacking the casing to the trimmer stud with an 8d finish nail beside the upper hinge, then check the hinge side jamb for plumb using a level. If the jamb is plumb, tight to the trimmer stud and the casing sits flush against the adjacent sheetrock wall surface, secure the hinge side vertical casing to the wall with 8d finish nails spaced approximately 8-inch apart and two 8d finish nails through the face of the vertical jamb into the trimmer stud even with each hinge.
Check the gap, called the “reveal,” between the edge of the top of the door and the horizontal jamb header piece to make certain it is between 1/8- and 3/16-inch and consistent from left to right, and the header casing fits flush against the adjacent sheetrock wall surface. Lift the latch side of the jamb slightly, as necessary to even the gap, then nail the header casing to the wall with 8d finish nails spaced approximately 8-inches apart.
Examine the reveal between the vertical latch side jamb and edge of the door, making certain it is between 1/8- and 3/16-inch and consistent from top to bottom. Secure the casing to the wall along that side with 8d finish nails at approximately 8-inch spacing, making certain the reveal remains consistent.
Insert a pair of shims between the latch side jamb and the trimmer stud on the opposite side of the split jamb near the top corner of the jamb to a point where the shims just touch the jamb, but do not press against it. Secure the shims in place with 8d finish nails, then use a utility knife to cut the excess shim that extends beyond the vertical edge of the trimmer. These shims provide backing support for both the installed split jamb assembly and side to be installed.
Repeat Step 8 to insert shim support for the latch side jamb at approximately 12-inch vertical spacing, making certain to install shims directly behind the latch strike location, as well.
Swing the door open and remove the center screw on from the jamb leaf of the top hinge. Replace the hinge screw with a 2 1/2-inch wood screw to penetrate the trimmer stud. This longer screw will help prevent the door from binding or sagging over time.
Attach the remaining split jamb assembly to the other side of the doorway, securing it in place with 8d finish nails through the jamb into the trimmer studs and through the perimeter casing into the adjacent wall. Make certain to keep both jamb surfaces flush as the assembly is being nailed and do not insert any nailing through the head jamb surface, keeping the header nailing confined to the exterior casing only.
Installation of the split jamb door assembly is now complete and the door is ready for installation of the lockset and strike plate following the manufacturer's instructions.