1. Undo the velcro fastener on the top strapPress and hold the quick adjustment button, and then gently pull until fully extended.
2. Position the headset over your eyes. While holding the quick adjustment button, slide the headset band until snug with the back of your head.source: https://www.vive.com/uk/support/focus3/category_howto/putting-on-the-headset.html
   
   
3. Turn the adjustment dial to fine-tune the fit. 
4. Adjust the top strap until snug, and then attach the velcro fastener.

source: https://www.vive.com/uk/support/focus3/category_howto/putting-on-the-headset.html