Think of what you want your poem to be about. For example, you can write a poem about a firetruck, your blue bird friend, or a cow named Sally. Choose a subject that makes a sound that can be imitated by a word, or several words.
Begin writing the first line of your poem. For example, you can write, "My little blue bird friend, wakes me every day.
Start sprinkling in some onomatopoeias into your poem. Below is an example:
"My little blue bird friend,
wakes me every day,
with his chirp chirp chirping.
His singing starts me on my way.
Today my little blue bird friend,
didn't tweet to get me started.
As I went to my window,
I felt so broken hearted.
I heard a pop, a crackle,
a snap, and a thud,
as I watched my little blue bird friend
fall into the mud.
My little blue bird friend had fallen
with a branch that was old and dead.
I now breath a sigh of sorrow
when it's time to get out of bed.
This is all because I miss the chirping,
of my little blue bird friend."
In the above example, "chirp chirp chirping" are onomatopoeias because those words mimic the sound that a bird makes. The words "pop," "crackle" and "snap" are onomatopoeias because they represent the sound a branch makes when it breaks. "Thud" is an onomatopoeia because it mimics the sound of the bird hitting the ground. "Sigh" is an onomatopoeia because it represents the sound that is made when a person is sighing.
Continue writing lines to your poem as you sprinkle in onomatopoeia throughout. Each line does not necessarily have to rhyme. It's a matter of personal preference. You can make the poem as long or as short as you like.