Here is a video by Fashion Sewing Blog on the fly zipper. It is a very good video that is easy to follow:
Here are a few other illustrations or tutorials:
Oliver and S fly front instructions.
Power Sewing Step-by-Step by Sandra Betzina, page 62.
Pants for Real People, Palmer and Alto, page 106.
Making Perfect Pants from New Mexico State University has a nice little chart with body measurements, ease, pattern measurements and the difference. Nice little way to come up with the amounts to alter.