In class we made a blues riff by asking and answering simple questions, such as, "what did you do today?" The answer could be "I'm sanding the car, getting ready for paint". That answer could be made into a riff that covers two measures. Another could be "gonna go home to see my gal". If you play these along with the first few measures of a 12-bar blues, you can make a riff that follows along with the words.