The Chuck Harris helmet-mount mirror is so sturdy that you can literally pick up the helmet by the mirror and swing it around without dislodging the mirror. Mounting it is pretty simple, but the geometry of the clamping part isn’t obvious until you actually find the correct orientation. So, I’ve tried to take some photos to make it a bit clearer.
These mirrors are made to be on the left side of your head/helmet for looking over your left shoulder — normal in right-hand-drive situations.
The first photo shows the clamping part the way you should see it when you go to put it on the helmet. To the left is the mirror stem; it should be on the outside of the left-front edge of the helmet.
Here’s another picture of the clamp part, with a piece of wood to show where the edge of the helmet goes. The lumpy part where the mirror stem attaches to the clamp part — that should wind up on the outside of the helmet, just above the edge, above your left eye. Actually a little left of your left eye.
I usually hook the inside part of the clamp (the loop that’s on the bottom in these pictures) on the inside of the helmet, then rotate the clamp so the lumpy part goes over the front of the helmet. At some point, there is usually a ridge of some kind in the helmet’s surface and the clamp will catch there permanently. My mirror doesn’t have the clamp flush with the surface of the helmet’s surface, but it is very firmly in place.
Below are photos of the mirror on my helmet, showing the orientation of the mirror and its position on the edge of the helmet. Click on the photos to see a large version, if that helps. Once you see how the mirror clamps on the helmet, it should be very easy to do it. And, chances are you won’t need to do it again until you get a new helmet.