Depends what sort of 'marshalling mask' youve got m8. On the flex range i've found the easiest way is to remove the face plate (by pulling it off..) then slowly but firmly pull one corner of the existing strap half way out of the slot. Then, while using the existing strap as a wedge to keep the slot open, feed one corner of the new strap through the same slot. then you can remove the old strap and pull the new one thru. Repeat on other side and replace faceplate/ear protection. Its not difficult and should take u no more than 5 minutes max.