Stuck this to the beaded chain of your blinds such that when you fully pull in one direction the blinds turn exactly to the open position and when you fully pull in the other direction the blinds turn exactly to the closed position.
On my office blinds, I had to install it only to adjust the opened position.
With the right dimensions, the stopper will snap to the chain and will stay there securely. If you want, you can add a drop of glue or heat it slightly and press gently with your fingers to close the gap. To remove you can simply cut it and print another one ;-)