bit of an odd question... are you asking where the material can come off on the zetec flywheel specifically, or how the process works in general?

if you're asking the former, the best way is to take material off so it looks similar to this: http://www.dunnellengines.com/images...20Flywheel.jpg

you can also trim down the rear of the flywheel for a little more weight saving, but don't touch the trigger wheel if you intend to use it. it would also be best to get it balanced afterwards as it will probably be a fair way out after machining.