Whether you're ready to tackle this kind of weld or not, I'll post an answer as it might be of help to someone else, or yourself when you get a bit more experiance.
The purge line goes into one open end of the tube, if there are a series of bends/outlets etc, then you put it into the lowest point of entry. There are many ways to secure it, some use foam discs, some use silicoln bungs, some wrap tinfoil/wire wool around the tube and some just tape it in place with aluminium or masking tape. Whatever you use, you will most likely need to tape over it, as the last thing you want is any air to be drawn in at this (or any) point. Next, tape over all joints (masking tape works best for this IMO), and weld them one at a time removing the tape only from the the one you are going to weld. The reason for this is, again, air can be drawn in from even the tightest fitted butt joints. At the highest point of exit (Argon is heavier than air) you need to make a small hole for the air to escape, about 6/8mm should do it. I don't know CFM, but normally put about 8/12LPM in for the inital purge, and then back it down to about 4LPM when I decide to start welding. Before you even start welding make sure you give the purge enough time to do its job, there is a formular to calculate this, but TBH if its only small tube (less than say 3" ), then a cigarette and coffee break will be plenty, although I normally check the purge by holding the flame of my lighter to the escape hole (not the most accurate method, I know).
I think that covers the basics, although this method refers to hygenic tube, and obviously others will have their own ways of doing things. Stainless pipe needs to be tackled slightly differently, and is a bit more involved.