In order to get a better picture of what a barrel roll looks like imagine flying a corkscrew loop. It should look like one turn of a very elongated coil-spring (figure below). Although the previous postings indicated how to perform the barrel roll, because each aircraft behaves very differently, you will have to work out the exact control inputs by yourself.
When I learned to perfom it I started out by trying to make my loops looking more and more like corkscrews. You will find that in order for the barrel roll to look nice you will have to alter the control inputs during the maneuver.

/Red B.