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Funerals have been part of the game since it was originally released. To have a good one, it's really no different from any other party. Food, music, games, etc. Make sure the Sims attending have a good time. You can also purchase the Legendary Host Lifetime Reward and it will guarantee that every party is a big success. The only real difference with funerals is that you'll get some different interactions, such as "speak kindly of deceased" and probably a mourning interaction.