**Note: This achievement guide is based off the One Giant Leap for (Bat)Man-kind achievement basically these two (plus Hey Diddle Diddle achievement) are located in the moon base -- links to all 3 are below.One Small Step for (Bat)Man...
Visit the Moon Base in the Hub
You need to talk to green loontern (daffy duck) he is located Inside the watchtower where the batman info signal is right outside the door to the moon base (the end of the video) after that build the door and walk in achievement pops right after.
Below is for the One giant leap for (bat)man-kind achievement ------ the original guide.
You can get this when you go to the moon in the base you will see the 6 doors to each of the lantern planets you will need to build the door using a fast character such as superman or the flash builds the doors quick all you need to do is go in and out of each of the doors in the moon base and the last one is oa green lanterns its directly behind the entrance / exit a big green door (need to build) as soon as you enter the last one achievement pops.
***note: You may need to beat the campaign past a certain level ie; to unlock the respective planets this will not be obtainable from the start of the game.
Video included for locations.
Links to Achievements: