The plane will take all players that get on the plane to the bridge.
After the electric chairs appear on the bridge, use them to return to the Prison. This needs to be done three times before step six. After flying the plane, it must be refueled before used again. Fuel will be located where the plane parts previously were.
To build the plane, you need to do the following steps
Get the Warden's Key
The Warden's Key is on a hook above the Warden's Office or above the Showers. To get it, one player must enter the afterlife, jump through the blue door above the office's door or above the Showers, and overcharge the generators. If it's above the Warden's Office, don't forget to charge the volt meter to open the door, as well. Overcharging the generators will lower the key, allowing any player to grab it for the entire team. The Key opens select locks around the map.
Get the Plane Parts
There are a total of five plane parts around the map. These parts can be collected in any order and built on the middle of the roof. It can even be done on solo! For those that don't know how to open the roof, head to the Infirmary and enter the Afterlife. Go through the blue door around the tiled hallway and follow the path up to the rooftop. Charge the volt meter near the door to gain access to the roof. The parts are listed below in sub sections:
This part is located at the bottom of the Docks. To the left of the M1927 is a locked gate. Use the Warden's Key to open this gate. On the other side is a Volt Meter and another locked gate. Have someone enter the Afterlife and charge this meter, then have another player enter the gate and pick up the part.
This is located inside the elevator in the middle of the spiral staircase in the Citadel Tunnels. This particular tunnel is just before the Docks. One player must enter the Afterlife and walk from the top to the bottom. A random set of numbers will appear on the walls on the way down, so the player must remember which numbers appear. At the bottom of the staircase are three volt meters locked in a box. Use the Warden's Key to unlock the box, then charge the meters when in the Afterlife to change the numbers above each meter. Match the numbers with the numbers on the wall to lower the elevator. Head down to the elevator to pick up the Rigging.
This part is located in the Warden's Office, but it won't be so easy to get. The gate it's behind is electrified. You will need to shut off the power before the Warden's Key can be used. To turn off the power, head down to the generator room near the docks. When there, enter the Afterlife to walk through a blue door to a smaller room. Overcharge the generator in the room, then overcharge two more highlighted generators in the main room. This should shut off the power to the door in the Warden's Office, allowing you to grab the Engine.
This part is located in the Showers room. At the end of the room is a gate that must be opened with the Warden's Key. When open, charge the Volt Meter to turn on the power to the washing machine. Interact with the machine to turn it on. When the cycle is done, interact with the machine again to get the Uniforms.
This part is located in a green, glowing box in the Infirmary, right before the Roof. Simply use the Warden's Key to open the box.
Build the Plane
After grabbing a part, it can be built on the ramp in the middle of the roof. Remember, to get on the roof, head to the Infirmary and enter the Afterlife. Go through the blue door around the tiled hallway and follow the path up to the rooftop. Charge the volt meter near the door to open it.
Get the Hell off the Island!
Once the plane is built, have all players interact with the plane from the back to get on. If they don't do it quick enough, they will miss the ride and the achievement / trophy! The plane will fly to the bridge where the Pack-A-Punch Machine is located.
Optional Step 5: Return to the Island
If you wish to go back to the Island, use the Electric Chairs that will appear on the bridge. Don't worry, all weapons and perks will remain intact.
Return to the Bridge
If you wish to return to the bridge again, the plane can be refueled by collecting fuel tanks at the same areas the five plane parts were located. This can be done as many times as you like.