Determine in what style and tone you will be writing. Will it be casual and breezy? Will it be formal and elevated? Determining the style and tone of the poem will give you a better sense of what diction to use.
Determine the theme, tone and mood of the poem. This will provide some ideas about what type of diction to use for the poem. For instance, if the poem is about death, then a more formal diction might be suitable. Then again, if the poem is a humorous take on a funeral, then a more casual diction can be used.
Choose vocabulary carefully. Pay close attention to the types of words that are being used. Determine the effect you wish to have on the reader and choose words that will enhance that effect. In a formal, classical poem, elevated or Latinate language may be more effective. Simple or colloquial language might be more effective in a humorous poem. Yet elevated language can be used in a simple or humorous poem as well, as long as it enhances the poem’s meaning, tone or mood.
Pay close attention to the syntax used in the poem. For instance, a poem can be composed of simple nursery rhyme sentences or in long sentences that spill over from one line to the next. It can also involve clauses that fold in on themselves. Again, use syntax to create an effect with the poem.
Use vocabulary and/or syntax to enhance the poem’s rhythm and pace. For instance, a poem that uses multisyllabic words and long sentences might create a breathless effect for the reader, while a poem using monosyllabic words or short sentences will create a faster rhythmic pattern. Read the poem aloud and determine whether the diction or syntax quickens or slows down the poem and determine which works best.