Here are some videos on the subject from a man who I consider to be an expert welder and teacher...
http://www.youtube.com/watch?v=bBXjRS4e6PA
http://www.youtube.com/watch?v=H9X30P6rrLQ
http://www.youtube.com/watch?v=RPlMCAibE8c
There are lots of pipe welding videos on youtube from others.