Choose a central idea about which to write. Then, write the first line of your poem. Select a rhyming word for the last word in the poem. Put this word at the end of the second line. You could also choose two words that rhyme first and write your poem around those two words. For example, you might write:
My daughter sat down to look
At the colorful pictures in her book.
A couplet tells a whole story, so if your story is more involved than just a couple of lines, write more couplets to put together to form a longer poem. The rhyming scheme would be A, A, B, B. This means that the first two lines rhyme, and then the third and fourth rhyme.