The jamb assembly is held in the opening between scraps of strapping. A temporary spreader near the bottom maintains a uniform finish opening. The author first cuts the side jambs to compensate for an out-of-level floor.
With shims at the header pinning the side jambs to the floor, the author screws the top of the hinge jamb to the framing.
A door-length level clamped to the jamb provides a reference for both plumb and straight, leaving hands free to insert and adjust the shims.
A finish nail through the jamb prevents the shims from loosening. They'll split readily around the nail if further adjustment is needed.