Decide the subject matter of your poem. It's a horror poem, but there are many different subjects you can write about in this genre. Decide whether you want to tell a story or just express a simple fear. Think about how you can use metaphor, symbolism and other literary devices to do this. You might just start with a title and see if that sparks ideas. Consider starting with a universal fear and build upon that--for example, fear of the dark.
Write off the top of your head. Don't think too much about the first stanza, just write down some of your feelings about darkness. Here's an example:
Black as ink, darkness surrounds me When I close my eyes GO And I feel as if nothing Will be there when I rise.
If you like what you have, build upon it. You don't need to rhyme. You don't need to count syllables. Just write free-form.
Develop the idea from your first stanza. Try using the word \"tomb\" as a metaphor for your room. Here's an example:
My tomb, my special place becomes my deepest fear. Things shift in the shadow, As they draw so near.
Write your ideas down in a notebook. Don't be afraid to scratch words and entire phrases out to make any changes that will add to the depth of your poetry. Try to capture the images you see in your mind when you think about your subject. Aim to scare your audience by writing something that will strike a fear within them. The best scary poems find a fear and bring it to the surface.