Achievements are created by going to the achievements settings page by going to Admin -> More Admin -> Achievements.
On that page you can create the possible achievements that players can get in your game.
To give those achievements to players at the correct time, use the achievement action in the effect builder.
To properly give achievements, you must do both. Have an achievement correctly set up and then give the achievement in-game.
Coming later: Achievements for out of game actions.