[ ] Design it in advance.
[ ] Write out a basic string of events/things in the setting.
[ ] Sketch out some maps that would fit the events.
[ ] Write down all of the primary quests to fit the events.
[ ] Based upon the sketch & quests, draw up a list of NPC types needed, inc. attackable, raidable ones.
[ ] Based on the list of quests, plus NPCs, determine how many items are roughly required for rewards and treasure drops.
[ ] Design some items which fit the setting.
[ ] Create item specs, using dummy attributes and artwork, for now.
[ ] Create NPC types, using dummy artwork, for now.
[ ] Create the maps roughly, using the correct tilesets.
[ ] Create the NPCs, inc. Quest NPCs, at the right coords on the maps.
[ ] Create the quest specs, having them reward the dummy items.
[ ] Create the NPC treasure lists, having them drop the dummy items.
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[ ] Draw the item artwork.
[ ] Draw the NPC artwork.
[ ] Draw the POI/map artwork.
[ ] Draw tileset(s).

Screenshot of incomplete tileset for an asteroid with crystal protrusions:
Crystals lacking shadows, rock is recycled for now, as it takes hours to make all of the interconnects from scratch.

Nice drawings

Screenshot of incomplete cave interior tileset:

Screenshot of incomplete grounded cargo vessel interior:
Boxes and tables recycled for now, because, well, anything to meet deadlines.

Screenshot of incomplete lava asteroid tileset: