How to unlock the 'Give Peace a Chance' achievement

if you have the 'Civilizations Expanded' mod installed, disable it. This replaces the key mechanic you need to have to get the achievement, Gandhi's Satyagraha.

Click on advanced setup and enable duplicate civs and duplicate leaders.

Make every player (including yourself) Gandhi. Don't worry about the exclamation mark at the side, because the game will automatically chose different colours for each player.

For those who have R&F and/or GS, you can set every AI player to either Poundmaker or Wilfred Laurier. The reason every player needs to be either one of these three is because they have the most peaceful AIs in the game.

After doing all of the steps above, you can click on 'Start Game' and continue playing as you normally would. Assuming that you do not end up in a war with a player that has founded a religion, the achievement should be yours sooner or later.