In order to use the jet pack in Call of Duty, you will first need to equip it. To do this, go into the customization menu and select it from the list of available equipment. Once it is equipped, you can use it by pressing the jump button while in mid-air. The jet pack will propel you upwards and forwards, allowing you to reach high places or cross large gaps. You can also use it to perform boost jumps, which are helpful for getting out of danger quickly or reaching ledges that would otherwise be out of reach. Be careful though, as the jet pack does have a limited amount of fuel and will eventually run out if used too much.