Click and drag the fireball to set power and angle, then release to hit as many torches as possible. Hit the required number of torches, and progress to the next level.
Press 'S' to suicide if the ball is not going to hit any more torches. The level will continue until all crates have finished burning in case they might light a torch. Pressing 'S' will end the level immediately.
Pressing 'R' will reset the level.
Pressing 'Q' will take you back to the level selection screen.