You'll need to
use Tiled to make the map itself, and I suggest
unpacking the game files and opening one of the vanilla interior maps to start from. You can use
TMXL to add a new location that can be referenced like any other map in things like warps, schedules, events, etc.
Daisy's Ultimate Farm has a simple TMXL pack that adds the additional locations the farm uses which you could look at for a stripped down example, though most popular NPCs on the Nexus have packs for their custom locations, spouse rooms, festival spots, etc.