Take the Venetian blinds down from the window with a small screwdriver.
Lay the blinds on a flat surface and remove the blinds carefully from their brackets.
Pry off the cord cover that secures the cord loop with a screwdriver. Pull out the old cord from the mechanism.
Install the replacement cording by pushing it through the gears of the cord loop mechanism.
Snap on the cord cover that you removed earlier back into place. Install the Venetian blinds onto the window. Test the cords to make sure they are operating properly.