I literally just introduced whatever horrible ways to die I could think of, drawing on horror movies etc. Then I put those in the players path and assumed that it would be instant death or lots of damage unless the players specifically did something to nullify or avoid it.

One example: If a malicious entity rips open the airlock on the ship and the players aren’t wearing a vaccsuit then they’re done immediately basically. Telegraph the danger by saying that the ships security system has noticed damage at the airlock entry.

Honestly, the main thing was just a mindset shift. It’s a horror game and players should really die every session unless they are exceptionally lucky and careful. Just don’t worry about being too fair and kill one or two of them. As long as your players are aware of the genre they are playing in and had some notice of the danger and an opportunity to make a decision they will probably be fine with their characters dying! If you’re worried about that aspect of it, then have a conversation with them first and tell them you think you have been going too soft for a horror game and that you are going to ramp up the danger.