Return to Monkey Island

This achievement can be unlocked during the "Part I - A Friendly Place" part of the story while you are playing as Guybrush. For this achievement, you need to be inside Wally's shop: "Wally's Maps-n-More." You can get here only after Guybrush fails to board LeChuck's ship: after exhausting all the dialogue with the quartermaster and returning to the docks, a scripted event will play with the Voodoo Lady talking to Curator Conrad about Wally leaving the Chamber of Commerce Midnight Luncheon early. Only then Wally's shop will be open. Once in the shop, you need to interact with all 18 maps, as shown in the screenshot below:

https://imgur.com/a/0MtNBQy