Ohh this is hard to explain
They do up exactly the same way as a motorcycle helmet
Pass the strap through both d-rings, starting with the ring that is closest to your leg(I think). Once you have passed it through the first d-ring, continue passing the strap through the second ring.
Separate the rings with your fingers and loop the strap back in, passing it through the ring closest to your leg.
Pull the strap firmly.
If you know anyone who has a bike helmet they can show you very quickly
Jason