With plenty of speed, point the nose up a bit, feed in aileron and hold it in, as it rolls inverted feed in down elevator to keep the nose up (push stick forward a bit), take out the down elevator input as it rolls level.

You can add some rudder in the direction of the turn to speed up the roll rate.

... or....

If you're a masochist and want to do a slow roll with your plane, you can feed it some rudder at 90 and 270 degrees in order to help keep the nose up. But plane will be fighting you if you do that because of the high degree of roll coupling inheritent the design of the plane. As you come through 90 you would need to feed in opposite rudder and additional aileron to counter that rudder input. Then as you come through 270 you would need rudder into the roll and less aileron since you're rudder is now increasing the roll rate. Elevator input is same as above. With a more capable plane you wouldn't have to worry about adjusting the aileron input as you feed rudder in and out.