To place your DUO band on your Original Grain watch you will need proper tooling to remove the pins from the Clasp. If you did not receive a resizing tool with your watch you may need to take the watch to a jeweler to remove the clasp.
To remove the clasp you will need to first open the clasp to have it stretched to its outermost point. You will then need to remove the pin from the closest link to both sides of the clasp. Once these are removed you can use the same pins to place both sides of the DUO in replacement of the Original Clasp.
Once this is done feel free to place your apple watch on the DUO Connection.
Please feel free to reach out if you have any further questions.