Remove the door, hinges, trim, and casing
Score paint and caulk along the frame edges
Pry out the old door frame carefully
Remove any shims, nails, and screws
Measure the rough opening
Cut the new frame pieces to size
Assemble the new frame
Set the new frame into the opening
Check that the frame is plumb, level, and square
Shim the frame as needed
Fasten the frame to the wall studs
Reinstall or replace the door stops
Rehang the door and attach the hinges
Test the door for proper swing and fit
Fill gaps with insulation or caulk
Reinstall trim and casing
Patch nail holes and touch up paint