Think of a person or place to write about or make up a humorous character for line one. Limericks create images so physical features work well. Feel free to play with language; it is common in limericks. Name this person and/or place in the first line. Either a complete sentence or simple phrase works.
"There once was an Old Man who supposed"
This is rhyme scheme A.
Continue developing the idea for line two. Rhyme the last word of this new sentence with the last word of the previous one.
"That the street door was partially closed;"
Now the poem reads AA. (The first two lines rhyme.)
Begin the third line with a new idea about the poem's subject that continues the story. The third line should measure about half as many syllables as the first two. End this line with a word that does not follow the AA rhyme scheme.
"But some very large rats"
This is rhyme scheme B.
Write the fourth line; it should have a similar length and rhyme with line three.
"Ate his coats and his hats"
Now the poem reads AABB.
Conclude the poem here with line five. This line serves two functions; it offers the "punch" of the joke and rhymes with lines one and two (AA).
"While that futile old gentleman dozed."
Now the poem reads AABBA.
The finished poem looks and sounds like this:
There once was an Old Man who supposed (A)
That the street door was partially closed; (A)
But some very large rats (B)
ate his coats and his hats (B)
while the futile old gentleman dozed. (A)