Since this post is dropping on Halloween I suppose I should discuss something scary. So let me start with this
which is a series of scenes from movies where someone explains to you how and why they work. I found it fascinating to watch because I love knowing how things work so I can apply them back to GMing. Fortunately I watched this just a couple days before running Elena Turduck and the Ghosts of Odysseus so I was able to put the ideas in here to good use.
Now some of the stuff in here you just can't do as GM - you can't control visual space. Sure, you can control where the players focus by what you choose to describe to them, but a lot of film tricks about setting up the geography and being asymmetrical in how you fill that space aren't viable. That just means you have to make the most of how you can make the focus. Make sure to mention the things you want them to visualize prominently, and then mess with them. Describe the chair being in the room, linger on it, include it in every explanation of the space and it becomes important. Then do something with it. or don't, if you want to keep ti creepy. As in weighted tones "is anyone sitting in the chair?" to watch them scurry.
Rely on light and darkness in your descriptions. In one sequence from last session Elena was alone in a big control room space - just her and the hum of the machinery in the glow of the control lights. She found the big old light switch and as the lights flickered on she saw shadowy humanoid shapes in each flicker, and with each flicker they moved further away from her until when the light became constant she was alone in the room. At that point you can't help but wonder what happens when you have to go somewhere dark...which the PCs of course had to do shortly.
Plus, that darkness was in a confined space - riding along on a conveyor belt through the bowels of the plant with a ceiling so low they had to lie down and machinery clanking on each side. Watching how each of then PCs entered that space gave the players a way to say something about them, and once they were so confined I could attack them or not. It's a open question whether which is scarier. Instead I had something happen to one of their NPCs that made him scream and never come our the other side.
Use non-visual senses. The best indicator that things were hunting them was the monkey-like fug of the creatures that the French Pastry Chef's delicate nose could detect. The most effective table jumps I got were when the monster chasing them uttered its hawk-like attack scream as it swooped down from the trees in a blur of motion. Deny them the visual in your description until its almost too late.
Focus on things other than what you're really showing. When one of their troops was slaughtered by an enemy construct off screen I didn't belabor the description of the body - I let the person who found him know he had been disemboweled and had his throat ripped but didn't go into detail there. Instead I focused the detail on the blood pooled on the jungle leaves 4 feet off the ground, dripping to dirt. That told them quickly how bad the death was, how strong the monster was and gave me a visual call back to bring up later. When one of the PCs finally got hurt I could come back this, describing the blood drip, drip, dripping to the ground. (It also turned out to be a salient clue to the plot, but I wasn't thinking that when I set up the first image. Learn to call back to yourself! It makes you look smart)
Every once in a while it's nice to scare them. Just don't go overboard (unless it's a horror game by design, but remember the real difference between fear and shock, or shock and disgust.) Have fun! Happy Halloween!