Correct Helmet fit
Your helmet should sit on your head so that it is Low, level, and snug.
To check your helmet look in a mirror and make sure that it sits level on your head, with the front of the helmet covering most of your forehead.
The chin strap should be snug beneath your chin. You should be barely able to fit a finger between the strap and your chin with the V point of the side straps siting just below your ears.
Check the helmet by attempting to push it back or forward. It should not move by more than an inch. If it moves to far back adjust the strap under your chin and if it moves too far forward adjust the lever at the back of the helmet. You should be able to move your head downwards towards the ground without it moving.
Important to Note: Helmet straps tend to loosen, so give your helmet a quick wiggle-check before every outing. There is a best before date on your helmet. It should be changed every 4 years.
This is a nice video that demonstrates how to fit your helmet correctly