Familiarize yourself with a traditional blues lyric. You'll see that while sorrow and heartache are staples of blues content, many of the lyrics are humorous. Early blues music often highlighted the troubles in African-American culture. Today, you can apply the blues form to just about any tragic event.
Choose your subject matter and write down some ideas. Literary devices such as irony, symbolism and metaphor are ideal for blues subject matter. Old blues musicians made use of these devices in many cases without even knowing the names of them. After you've found your subject, think of ways to present it using one or more of these devices.
Write two lines of equal length that set up a problem or introduce a hard-luck story. Write a third line that expounds upon the first two lines. Here's an example:
My baby walked out that door. My baby walked out that door. My baby walked out and now my broken door won't open up no more.
This example uses the door as a metaphor for the narrator of the poem. The last line uses the word as a metaphor for the narrator's heart.
Continue to build verses based on the same old-school blues structure, with each verse introducing a larger problem. Don't be afraid to make the lines funny. You can also bring in modern ideas. Here's an example:
I had to walk back to Texas. I had to walk back to Texas. I had to get on my feet cuz my baby she took my Lexus.
It's simple and it's fun.