There are two different requirements for each of the endings: having a clean room (room 302), saving Eileen at the final battle, letting Eileen die at the final battle, and having a possessed apartment.

To save Eileen: At the final battle, kill Walter before Eileen walks to her death. Note: The healthier Eileen is, the slower she walks. See the section on Eileen's health for more info.

To clean room 302: Use a holy candle or saint medallion on every spirit in the apartment. You should get rid of each spirit as you see them. See the section on spirits in the apartment for more info. Note: The apartment must have been kept 80% free of spirits for it to be clean.