You could write a poem telling how to eat spaghetti, how to ask for a date, how to sharpen a knife, how to let go of grief, or even how to write a poem. Or you could take it to extremes: how to prevent global warming, how to kiss a snake, how to become dust.
This can be serious or lighthearted, genuinely helpful or intentionally misleading. It may help to assume that your reader is completely naive and has no understanding of the process. William Carlos Williams's "Tract" could be considered a "how to" poem, as could David Wagoner's "The Principles of Concealment."
Keep your poem between 9 and 21 lines long, with at least two stanzas.