Undertale

This trophy (and all other Dognation trophies) can be obtained by donating various amounts of G to the Dog Shrine. The shrine is located within Sans' and Papyrus' house in Snowdin Town.

To access the shrine, you must spare Papyrus during your battle with him after reaching the end of the forest/town area. After this, return to Snowdin Town to see Papyrus standing outside of his house. Speak with him, and he will unlock the front door. Once inside, walk straight ahead into the kitchen and inspect the tall sink. After a short scene plays, walk into the newly opened doorway to find the Dog Shrine.

You can only donate a set amount of G to the shrine at any given time before you must leave the room to see how the dog spends the money you've given it. A total of 350G is needed for all levels; for Level 3, only 6G is required, and this unlocks an inflatable waving mascot in the shrine room.

Assuming no G has been spent by the player up to this point, and some areas/minigames have been grinded (you can replay a golf-like game in the Snowdin area for G by pushing a snowball in to a hole; the better you perform/the faster you do so, the more G you receive), it's possible to have all 350G already, just by sparing monsters and examining various objects (notably, the couch in Sans' and Papyrus' house contains 20G, and a snow poff before fighting the Greater Dog boss contains 30G). If not, you can return to the Dog Shrine at almost any time in the game after progressing further. Items can also be sold at the Tem Shop in Temmie Village.