Measure the existing door, frame, and rough opening
Choose a replacement door that matches the size and swing direction
Remove the old door from its hinges
Remove the old trim and casing
Pry out the old door frame and threshold if replacing the full unit
Clean the opening and check for damage
Repair any rotten or damaged framing
Dry-fit the new door and frame in the opening
Shim the frame until it is plumb, level, and square
Fasten the frame securely through the shims
Recheck the door operation before fully tightening
Install insulation around the frame
Reinstall or add exterior trim and casing
Seal gaps with caulk
Install the threshold and weatherstripping
Hang the door if it was removed
Install the lockset, deadbolt, and hardware
Test the door for smooth opening, closing, and locking
Paint or finish the door and trim as needed