For this achievement you will need to enter the specific code before the main menu. When the game starts up you will come to an autosave notification and as soon as you press '
continue' the letter C will start to form, so a few milliseconds after pressing continue that is when you want to enter the code.
I will list the codes for each platform below:
Xbox: Up, Up, Down, Down, Left, Right, Left, Right, B, A
Playstation: Up, Up, Down, Down, Left, Right, Left, Right, Square, X
PC: Up, Up, Down, Down, Left, Right, Left, Right, Escape, Enter
Switch: Up, Up, Down, Down, Left, Right, Left, Right, A, B
If you need more help with this you can see my video below: