This must be done in Forge.
First, have one player (Player A) spawn a Mongoose and make them get in the driver seat, then you (Player B) hop in the passenger seat. Have Player A return to Monitor mode, and push you outside of the map. You will probably die.
Return to the passenger seat of the Mongoose, Player A spawns a teleporter, sender node, on top of the small rock, to the left of the sea behind Red Base. Have them spawn a receiver node and push it out to sea, and then grab the Mongoose, and hold it above the sender node. You then get off the mongoose, and teleport to outside the map. Then, change to edit mode, and you will be able to fly to wherever you wish.
Note: If you fly too far up the Forerunner wall, you will be killed by a death barrier.
Method 2 Edit
This, again, must be completed in Forge.
This only requires one player. First, fly to the edge of the map, behind Red Base. Then, spawn a receiver node, and push it as far as away as possible. Then, delete it. Now spawn a sender node and place it at the edge of the map. Fly to the centre of the teleporter, and make sure you are facing where the direction you want to go. Then, spawn a receiver node, and you will be teleported outside the map.
Again, the same rules apply, the Forerunner wall has a death barrier.