Our pen-grip mice should be held using just the thumb, index and middle finger of your right hand.

Your thumb will rest on the left grip of the mouse pointing away from you. While your middle finger should sit on the grip opposite your thumb, just behind the scroll-wheel.

Since the mouse is so light these two fingers alone should give you precise control over its movements without needing to grip firmly. 

Once you have this grip your index finger should rest on the top of the front mouse button. But also be able to easily shift back to the rear button/s without changing your grip.

If you have trouble reaching the rear buttons comfortably, you may need to move your thumb slightly further back.

There may be a slight adjustment period to using this grip. But it should quickly start to feel natural and comfortable to use, just like holding a pen.