Graphics set for Dwarf Fortress v50 and above
Copy the mods into <dwarf fortress dir>/mods/ and enable them when creating a world.
If you want to use them in a preexisting world, or you just don't want to enable it as a mod each time, you can replace the vanilla files with the mod files.
mods/ribbitgfx_buildings/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_building_graphics/graphics/mods/ribbitgfx_creatures/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_creatures_graphics/graphics/mods/ribbitgfx_descriptors/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_descriptors_graphics/graphics/mods/ribbitgfx_environment/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_environment/graphics/mods/ribbitgfx_interactions/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_interactions_graphics/graphics/mods/ribbitgfx_interface/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_interface/graphics/mods/ribbitgfx_items/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_items_graphics/graphics/mods/ribbitgfx_plants/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_plants_graphics/graphics/mods/ribbitgfx_world_map/graphics/-><dwarf fortress dir>/data/vanilla/vanilla_world_map/graphics/
Additionally, you will need to copy mods/ribbitgfx_environment/art/ and/or mods/ribbitgfx_interface/art/ into <dwarf fortress dir>/data/art/ if you plan on using them, as the black background of the game and parts of the interface are unable to be modified outside of the art directory.
You should probably back up the art and vanilla directories before you overwrite them, though.
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