Remove the old door and trim
Check the rough opening for plumb, level, and square
Measure the opening to confirm the prehung door fits
Remove the prehung door from its packaging
Leave the door in the frame
Place shims at the hinge side of the opening
Set the prehung door into the opening
Center the frame in the opening
Check the hinge side jamb for plumb
Shim behind each hinge location
Fasten the hinge side jamb with nails or screws
Check the top jamb for level
Shim the latch side jamb
Close the door to check the reveal
Adjust shims until the gap is even
Fasten the latch side jamb
Fasten the top jamb
Test the door swing and latch alignment
Trim the shims flush with the wall
Install the casing trim
Caulk and finish as needed