Remove the door, hinges, trim, and casing
Score paint and caulk along the frame edges
Pry out the old door frame carefully
Remove any remaining nails, screws, and shims
Measure the rough opening
Check for damage, rot, or uneven framing
Repair or replace damaged studs or sill parts
Assemble the new door frame
Place the new frame into the opening
Shim the frame until it is plumb, level, and square
Secure the frame with screws or nails
Recheck alignment as you fasten it
Rehang the door and test the fit
Adjust shims or hardware if needed
Insulate gaps around the frame
Reinstall trim and casing
Caulk seams and nail holes
Fill, sand, and paint as needed