The center button opposite side of the ear cushion turns the headset on and off. Push it to turn the headset on, and you'll see a flashing blue light. Push it again to turn the headset off.
On the back of the headset there's a toggle switch. By toggling up, your listening volume with increase. By toggling down, your listening volume will decrease.
If you want to mute yourself, push the toggle switch into the headset.