Read the poems of some of the Transcendentalists. These include Thoreau's "The Summer Rain," Emerson's "Berrying" and Whitman's "Leaves of Grass." Study the overarching themes in the poems. Transcendentalist poets wrote about how nature uplifted them and connected them to something greater than themselves. They celebrated the beauty of the natural world and of human nature. The notion of an "over-soul" was a Transcendental ideal, which Emerson discusses in detail in his essay of the same name. The Transcendentalists believed that there was a force that tied all people and nature to one another.
Take a long walk in the woods; take a notebook along to jot down thoughts and ideas as you go along. Thoreau lived in a cabin on Walden Pond to get closer to the natural world and was inspired by the things he saw. Emerson attempted to see the world as a "transparent eyeball," perceiving all in nature, including human nature, that he possibly could. Let nature serve as your inspiration. Think about how the things you observe uplift you and write down how you feel.
Begin a rough draft of your poem. Don't dwell on how well you write at this point, but let your thoughts flow onto the paper. Draw inspiration from the walk you took. Think about what makes you an individual, your own uniqueness, but also about the things that tie you to the human race. Think about how being in nature made you feel and how nature can uplift the soul. Consider the beauty of nature and the goodness of human nature. Consider, also, the over-soul and the bonds that tie humans and the natural world together.
Fine-tune the details of your poem. It does not have to rhyme or even adhere to stanzas. Never sacrifice meaning or purpose for rhyme scheme. Thoreau's "The Summer Rain" employs rhyme, but Emerson's "Berrying" does not. Use whatever form suits you. The main thing to keep in mind is the content of the poem.