Problem with GNU radius and cisco privileges

I am sorry for anxiety, but to access any more to whom. 
I the beginner in GNU Radius. A situation such:
Is Cisco 1721 and a server with installed GNU Radius. Simple authentification works.
How to customise GNU Radius that it depending on the entered user name and the password on Cisco, distributed appropriate privileges from 1 to 15. I know that it is necessary to specify Cisco-AVPair = "shell:prv-lvl=15", but here in what precisely configuration file and what correct syntax of this command, I cannot understand. What strings, and it is necessary to register in what configuration files, that all has correctly earned in my situation? I have studied a manual on GNU radius, but the answer to the question have not found. 
Let will seem silly to set such questions, but more nobody could give the intelligible answer.
In advance to you it is grateful
                                                                                                B.Alexey