TBH, I've never used a helmet lock before. In my hometown if you did, your helmet strap was usually cut off by some a**hole passing by, and helmets there were more expensive than they are in the USA. So this was the first time I've actually used a helmet lock.
- Underneath your seat, there's a 5mm allen key. Mine's got some tape around it to make it easier to lift out.
- Use that to open the toolbox (counterclockwise = open)
- Inside there is the cable for your helmet.
- You can either pass it through the d-clip of your helmet and then pass it under the wire, or do it as three separate steps. I do it as 3 separate steps because I can't see what I'm doing if I do it as 1. YMMV.
- First loop under the end.
- Free end through the D-clip of your helmet.
- Second loop under the end of the clip
- Close the seat.
Once the seat is closed, the helmet is attached to the bike. See attached photos (numbers align with above list) for a demonstration. It's also in the owner's manual under "Storage equipment" although it is light on details.