Bend over a little at the waist. Keep your feet shoulder length apart, your legs straight and your head tilted slightly up so you are looking straight ahead.
Hold out your arms as if you were imitating an airplane. Have one arm farther forward and up, above shoulder height and have one arm farther down and back, below shoulder height.
Begin to turn. Stay bending forward at the waist and spin in a circle. Shuffle your feet as much as you need to. You may need to try this before holding your arms out to get used to motion, as it is not an easy turn.
Practice the turn for a bit until you can spin smoothly, at a constant rate and keep your body bending forward. This may take a few sessions.
Rotate your arms as you spin so that each time you spin around, your arms are at a different height. If you were standing up straight and not spinning, you would look like an airplane turning left and right quickly.
Put on some jazz music. Choose a nice even beat that is not too fast or too slow or too improvisational. The closer to big band jazz it is, the better.
Try and spin on the beat. First, keep tempo, but eventually you would like to end each rotation of the barrel turn right on beat. This move can look chaotic up on stage, but it should still be rhythmic.
Stomp one of your feet at the end of each rotation to emphasize the beat. This will help you keep the rhythm and add a little flare to the move.
Work in a barrel turn or two when you are dancing to jazz from now on. Like any difficult dance move, it will only get better each time.