Think of a memory, image or something that really stirred your emotions. Keep that emotion in your head as a guide for the poem.
List the rhyming pattern in the left margin of your paper, with one letter or letter and number per line. The pattern is A1, b, A2, skip, a , b, A1, skip, a, b, A2, skip, a, b, A1, skip, a, b, A2, skip, a, b, A1, A2.
Write two lines that rhyme. Just focus on those two lines, which will stand for A1 and A2 in your rhyme pattern. The reason the "A" is capitalized is that these lines repeat exactly each time they appear. Now, you've only eleven lines to go.
Open a thesaurus and rhyming dictionary to help you craft the other lines. All lines designated with a small "a" have to end with a word that rhymes. All words at the end of "a" lines will rhyme with the last words in "A1" and "A2" lines.
Fill in the "b" lines. All words at the end of "b" lines rhyme with other "b" lines, but don't rhyme with anything else.
Go back over the poem as many times as you need to fix it up and make the image and emotions clear.