It's easy. There are some semi circular cut outs in the visor which locate on some round lugs underneath the cowls at the side of the helmet. Reattach with the visor in the up position over these lugs. Then shut the visor gently. It will be stiff and click and crack but this is normal and is only the visor locating properly. Open and shut the visor a couple of times to make sure it is in properly.
It's difficult to explain in writing. Bring it round to my house and I will do it for you
