Helmets often feature an adjustment wheel on the rear of the helmet. Opening the wheel fully will loosen the retention system allowing you to put the helmet on. Then tightening the wheel or dial will restrict the retention system, snugging the helmet onto your head.
Buckle the chin strap and ensure the straps form a "V" under each ear and sit comfortably. You can adjust the buckle under each ear to achieve a better fit should the straps not sit right.
With the retention system tightened, and the chin strap buckled, open your mouth wide to ensure precise fit. If the helmet snugs against the top of your head when your mouth is wide then the helmet is a great fit. If not you may need to tighten the chin strap to ensure the helmet stays in place during an impact.