How is rudder used in a aileron roll?
The key in flying a good roll is to keep the aircraft's nose up in anticipation of the lift requirement when flying in knife-edge or inverted flight. Since in knife-edge the elevator and rudder will assume each others function, you will need rudder as well as elevator for directional and pitch control. Also, depending on adverse yaw, some rudder is required to hold yaw to zero when applying ailerons. The procedure is as follows:


Speed up, so you are fast enough for roll maneuvering and inverted flight.
Pitch up a few degrees. How much depends on wing incidence and zero-lift angle of attack. Look out for some feature on the horizon: This will help you to keep your direction constant. 
Stop the pitching motion, then apply full ailerons. Correct the aileron-induced yawing motion by applying gentle rudder (trailing edge in the direction of the up-going aileron). Your goal is to keep the fuselage pointing at the chosen feature throughout the maneuver.
With increasing roll angle, apply rudder to keep the nose up. At the same time, push the stick gently to avoid course deviation. At 90° roll angle, both elevator and rudder have changed their function: The elevator will now control direction, and the rudder will control pitch.
Keep the rotation rate and push the stick more when changing from 90° to 180° roll angle. In inverted flight you need to keep the aircraft trimmed, and depending on the static stability and speed this can require considerable negative elevator deflections. At the same time reduce rudder deflection such that the fuselage will still point into the initial direction. At 180° roll angle both elevator and rudder have returned to their old function, but pulling will now start a dive.
On the way to 270° you will again need to add some rudder, now in the opposite direction, to keep the nose up. Again, elevator and rudder will exchange their functions.
Once you return back to normal flight attitude, stop the rolling motion and reduce rudder deflection back to zero.