First go to a map where there are Warthogs and forge there with two players or more.

Second, bring up the forge menu and spawn a warthog and then erase it.

After that have another player start a new round and when it comes up to ask if you really want to, then one player presses A and the other presses X at the SAME TIME and then when the screen goes black press A two times (this will also work with the scorpion).