In Act 1 - Chapter 0: Prologue, reach the section where you must defend the House Of Sovereigns on E-Day. Kill the Corpser with the Hammer Of Dawn to view a cutscene. Defeat the wave of Locust soldiers and Boomers that spawn afterwards. A Locust Reaver will land after this wave. Once the Reaver lands, kill the pilot of the Reaver, then quickly run underneath the Reaver and press X to climb on and control it. Note: If you want too long, the Reaver will automatically die. While riding the Reaver, you can fire rockets and kill the other Reaver in the area. You can also mount and dismount as often as desired.