1. Fully charge your phone

2. Start the app then options / activate (or reset if you did not origionaly follow step 1)

3. Use your phone as you would normally

The app will say calculating until your battery meter has dropped 2 bars.
Its important you dont try and hurry it up by trying to drain the battery.

When the app starts displaying remaining battery life, it is not displaying the standby time, it is displaying how long the battery will last based on your individual phone usage. Light phone user will get a much longer reading and vice versa.

On the matter of the Screen Saver, dont bother using it. all these screen savers drain you battery far faster than the standard clock/date saver.
For example I tested it using the Nokia Energy Profiler which gave the following reading.

On the standard screensaver my Nokia N82 on standby from a full battery will last 96 hours but with iOn BatteryTimer screensaver that goes down to 70 hours. This was based on two one hour tests.