Keep it simple. Focus on one creepy idea. For example, start with a normal situation like 'I was reading a book at night' and then add a terrifying twist like 'The words started bleeding off the page.'
Well, here are some. 'I woke up to find a handprint on my window. It was on the inside.' And also 'My sister said she saw mom in the attic. Mom has been dead for years.
One: 'The old doll in the attic blinked. I ran out of the house.' The horror here is that dolls are often creepy, and a blinking doll gives a sense of the inanimate becoming alive, which is very unnatural and terrifying. It violates our perception of normal dolls, and the reaction of running out of the house shows the extreme fear of this abnormal situation.
They often use common horror elements. For example, like the idea of something abnormal happening in a normal setting. In 'The picture on the wall changed. No one else noticed.', the normal wall picture changing is strange, and the fact that no one else noticed makes it even scarier. It makes you feel alone in this abnormal discovery.
The brevity of '2 setence horror stories' forces your mind to fill in the gaps. In 'I heard a knock. No one was there when I opened the door.', your mind immediately starts creating scenarios of what could be lurking outside. It's like a quick jolt to your fear sensors.