Scrub (move the current time indicator) to where you want the value to be zero. Hit the stopwatch for the effect in the effects panel, or for the property in the timeline.
Scrub to where you want the value to be 100 and then change the value for the effect. A keyframe will be added automatically.
Scrub ahead again and change the value to 0. Once again, a keyframe will be automatically added.
If you want the value to remain steady for a while (e.g. to stay at 100% for a certain number of frames) add another 100% keyframe at the desired time by hitting the small button at the left of the timeline panel between the two small arrows.
Simon Bonner
