User Exit For The Z Reports Problem

Hi,

I am have been assigned a User Exit for the Z Report with the Tcode but when i run that Tcode in my program which tells that there is no user exit is dere for this code ,then how should i made the User Exit.

For Example, the tcode contains the Conformation No.. similar to the tcode CO11N and there is a Usage Decision no.. similar to the Tcode QA32.

Till now i am able to have the information that i can have the User Exit of the those tcode which are the SAP Standard Programs,and put exit on them by modifying it through the tcode CMOD. But what about the

User Exit which consist of the Z Program developed by the me or the other ABAP Programmer.Bcoz at present there is no such User Exit available . so provide me guidlienes in solving this doubt..