Right where the helmet tether comes from there is a little hook. If you hook the loop from the end of the tether on that, then you create a loop thingie. So, you open the seat, put the tether through the d-ring on the helmet, hook it to the hook and close (lock) the seat. Now your helmet will stay on your bike and it'll take a determined thief (who brought bolt cutters for the tether) instead of just some guy who goes "Ohh helmet!" and ganks it off of your handlebars (assuming you'd leave it on the handlebars).
/I wouldn't recommend riding with a helmet on the tether though, it'll bang around and get scratched up.