Read dark poetry and highlight examples that you admire. Ask yourself what you like about the writer's style -- it could be an interesting metaphor, his word choice, the poem's rhythm or something else altogether. Recognizing what makes a dark poem effective makes your own writing better. If you're not sure where to start, try Sylvia Plath, Stevie Smith and Edgar Allan Poe.
Jot down words that capture the feeling you want to describe -- not whole thoughts, just words. If you're writing from the perspective of a character other than yourself, sit with your eyes closed and try to feel what that person might be experiencing before beginning this exercise. Your list could include nouns, adjectives, verbs and adverbs, but focus on choosing words that convey the spirit of your poem as accurately as possible.
Go for a walk and try to see the objects around you differently, keeping the dark message of your poem in mind. For example, maybe you could see that abandoned tricycle at the side of the road as a symbol of the fleeting nature of childhood. This exercise may feel depressing, but it can help you develop interesting metaphors for your work.
Choose an overall focus for your poem, using the metaphors and words you've written down in past exercises as inspiration. A poem about death in general can quickly start sounding cliche. Settling on a persona guides your writing efforts and results in a poem that sounds more authentic. Perhaps you're writing as yourself contemplating that bicycle on the road, or maybe you're a young man having a war flashback.
Write the thoughts that come into your head while keeping your overall focus in mind. Forget your reader for a moment, and instead just say what "feels right" based on the context you've chosen. For example, someone in the midst of a flashback wouldn't explain that fact to the reader. Just write what you think the character would think or say, then leave interpretation up to the audience.
Read what you wrote while considering the variables of meter and rhyme. If you want to create a sense of harmony or consistency in your poem, you could choose to edit your poem to include rhyming couplets and lines with specified numbers of syllables. If you want to communicate chaos, it generally makes sense to avoid consistency. Dark writing could have some parts written in one style and other parts written another way, with your poem starting out well-ordered and dissolving into confusion, for example.
Read your poem with consideration for line breaks. If there are words or short phrases you want to emphasize for emotional impact, putting them on separate lines or suddenly starting new lines unexpectedly can help draw the reader's attention to these features.