Wizard,
Since you are apparently an amateur, you should be made aware that rebarreling a Luger without the proper tools will likely result in a butchered Luger receiver... it is not recommended for the home gunsmith...
If you have the proper tools to take the old barrel out without marring the receiver or bending the receiver fork, and the knowledge on how to thread a replacement barrel made from a blank, then milling the extractor slot should not be a mystery to you. Just copy the barrel that you take out.
If you have not done this before on a Luger, I would have one of the craftsmen that are also members of the Lugerforum replace the barrel for you.
Hugh Clark (Hugh) and Tom Heller (lugerdoc) come to mind... but there are others who do this successfully on a regular basis and they are welcome to post a reply and tell us all that they are available for that kind of work, and what they charge. Some of them probably have ready to install 6 inch barrels to boot.
Here is what the barrel shank looks like when removed from the receiver:
