Think about a dozen situations that would terrify or disturb you. Pick the scariest of those and think about what would make them even
scarier. Think of ways you can innovate those situations. Think about realistic ways one could end up in such a situation (I find that many of the worst scares stem from the realization of '
that could happen to me'). Write down what you come up with.
That might sound really obvious and contrived, but it doesn't mean it's not effective

For real - imagine horrible scenarios as vividly as you can. If you're having a hard time thinking of scary situations, read the news, read National Geographic, read other horror stories, read books on the unexplained and the supernatural.
Being able to effectively build fear is being able to make your reader
believe in the story you're telling - you can get as fantastic as you like with your characters and actions, but make sure it sounds real. To be scared is to identify with a fearful situation, so you really need to make sure your readers can identify.
Obviously, take what I have to say with a grain of salt. All writers have different ways of writing and different opinions on what constitutes a good story. Personally, I like to make it possible for the reader to hear, smell, see and taste everything going on in the story. Others like to get in and out with vicious jabs that catch you unaware in the gut. Whatever works for you. Horror writing is still writing, so do what a writer does - recognize emotions and responses you want to get from the reader and choose your words to manipulate those emotions.
Good luck
