If you are looking for some great fundamental details on teaching the Reach Block, check out the video by Dan Roushar, currently the offensive line coach of the New Orleans Saints. The video was made while he was the OL coach at Michigan State. Coach Roushar does a great job of breaking down the first three steps as well as the position of the hands and head in executing an effective reach block that will lead to a great drive block.
This clip is just one of several good teaching points from the the DVD entitled Techniques & Drills for Creating Championship Offensive Linemen (Click the link for more info about the dvd)
The video has sound, so please make sure that you have your sound turned on. If you are watching from school, keep in mind that many schools block access to YouTube.