Devise a rhyme scheme (or choose not to rhyme at all). Decide the number of lines you will have per stanza and then mark the lines (physically or in your mind) a-b-a-b-c-c or whatever scheme works for you. The lines ending in the same letters rhyme with one another. Establishing a rhyme scheme early will help establish your form. Your poem doesn't have to rhyme, but if you choose to write unrhymed, limit the rhymes you do use. Delivering rhymes on some occasions and not on others can break the musical flow of your poem.
Choose subject and setting matter that will fit the supernatural theme. Ghosts are good, but anything outside of natural can fit the theme of a supernatural poem. Death is a popular overriding theme in supernatural poems, as well as returning from the dead. Settings might include Gothic environments, cemeteries, sanatoriums, castles, and even places that might not ordinarily seem spooky, such as a plantation house on a sunny afternoon. In fact, the unexpected setting can often be more effective because it's a surprise.
Choose words that evoke creepy images or by the way they sound, help create a mood. Here's an example:
Whispering wind breathing cold
Down my back as I move
Through darkness, seeking the
Sound of something breathing in
The darkness
The repetition of the "s" sound in this stanza, along with words such as "whispering" and "breathing," evoke the feeling of something waiting nearby, breathing in the darkness and waiting to strike.
Build suspense throughout your poem. Start descriptively in the first stanza (verse). Add more tension with each stanza by building upon the previous. Think of stanzas as chapters of a book, or a beginning, middle and end. Start by setting the scene and establishing a story, use haunting language and vivid imagery, and bring the poem to a surprising or unsettling ending.