Instructions
Plant a friend on the opposing team. For example, if you select RED, your friend should select BLU.
Ask your friend to enter the game as an Engineer, while you enter the game as a Spy.
Have your Engineer friend build a teleporter exit in his team's spawn area.
Meet your friend somewhere on your current map where you are not likely to encounter other players, and have your friend build the teleporter entrance. Spies can use enemy teleporters.
Use the teleporter to get beamed beyond the entry barriers and appear in the enemy spawn.