start grinding, when you have a pile of "steel dust" below your grinder thats about 3 feet tall, you will know how to flat grind.
Kidding aside, ive never used any videos, or even had anyone "teach" me, but I would say the video by Harvey Dean would be a good place to start if your looking for info. on flat grinding.
Harvey does exceptionally clean grind lines, among the best there is.