As days progress, eerie things happen and people grow fearful. Some people die in gruesome manner. Communication to the outside is cut off. The captain valiantly keeps writing, until death is immediately at hand. You can then optionally place the people in the present, who have just finished reading the journal, in the same peril.
Writers write about what they know, which is why so many characters are writers.
Naturally the characters tend to have an extreme dedication to recording what happens, even as the seven-eyed, tentacled, pincered creature is about to fly in the bay window and devour them. Instead of running away screaming, they write about the horror they're experiencing. The business of living a sort of recorded first person point of view breaks down when the writer dies. Wind has knocked down the power lines, and I write by the wan light of one guttering candle. It is released in this world, and it is coming for me.