so I give them a fline say f: mydydns 1200 abc 123 then they use that as there cline and they send me there fline which I enter as a cline in cccam.cfg?
No, your fline stays in your cfg file and goes to no one else. For example
F: USERNAME PASSWORD 2 0 0 { 0:0:2 }
C: (Their cline goes here, that they provided you)
You change the USERNAME and PASSWORD to anything you like, if for example i pm you. You could change the USERNAME to Daf (my name) and a random password.
Once you have the fline done, and have signed up for dyndns your cline will be made up of all these. E.g
C: jimc.dyndns.org 12000 USERNAME PASSWORD no { 0:0:2 }
As i say if you read the thread again that ten posted just to get used to the other strings on the end.