Start up a scenario.
Make it a 4 player hot seat (4 humans). Doable by pressing the Y button on the scenario screen (To choose advanced options)
Make the world small, and I'd honestly suggest you turn off all factions (spacers, psi fish ect)
From turn one, get all players to have most of their units auto exploring.
Don't bother too much with building or tech, just end your turn.
As you meet each other (your other selves), pick one of the players to be overlord. Under the diplomacy options when talking to another player, demand "you become overlord". Demanding the vassal option, oddly, makes YOU the vassal.
Do this with 3 of the 4 players and the achievement will pop the moment the 3rd accepting of overlord status happens.
Its an easy achievement, the only real struggle is finding the other players.
Happy hunting (>*_*)>