All maps have their own code file that is in the base of the _maps directory. Maps are loaded dynamically when the game starts. Follow this guideline when adding your own map, to your fork, for easy compatibility.
The map that will be loaded for the upcoming round is determined by reading data/next_map.json, which is a copy of the json files found in the _maps tree. If this file does not exist, the default map from config/maps.txt will be loaded. Failing that, BoxStation will be loaded. If you want to set a specific map to load next round you can use the Change Map verb in game before restarting the server or copy a json from _maps to data/next_map.json before starting the server. Also, for debugging purposes, ticking a corresponding map's code file in Dream Maker will force that map to load every round.
If you are hosting a server, and want randomly picked maps to be played each round, you can enable map rotation in config.txt and then set the maps to be picked in the maps.txt file.
Anytime you want to make changes to a map it's imperative you use the Map Merging tools