Choose a letter from the alphabet. Begin with a letter with plenty of words to choose from, such as S or T.
Think of a noun that begins with this letter and write it down. It's easiest to start by choosing a being instead of an object, like a person or animal or even a name. For example: If you chose the letter S, you could use "shark" as your noun.
Add an adjective to describe the noun that begins with the same letter or sound and write it down. If you have trouble coming up with one, look in the dictionary for words that begin with your chosen letter. For example: For "shark," you could say "shapely."
Write down a verb that begins with the same letter or sound. Again, use the dictionary for help. For example: For "shapely shark," try "shimmy."
Arrange your three words into a sentence, adding articles, tense and plurals to clarify the meaning. For example: you could write, "The shapely shark shimmies" or "The shapely sharks are shimmying." You have your first basic line.
Repeat Steps 1 through 5 for different letters and sounds. Keep in mind that eventually you will want to come up with several lines that make sense together, but allow yourself to brainstorm and you might come up with some surprising combinations.
Continue writing lines until you have enough to create a poem of your desired length and arrange your lines in order to create a basic poem.
Add additional words such as objects, adverbs or additional adjectives. If you want to use a specific word for meaning, but it doesn't fit with your alliterative scheme, use a thesaurus to find an appropriate substitute. Don't worry if your alliteration is not always perfect; the sound "sh" can be combined with "ch" and "s" to create partial alliteration, such as in, "The shapely shark shimmies on the shaded chalky street."
Read over your poem, correcting any grammar or spelling errors.
Write your final draft. Don't forget to give it a title.