Using CMOD procedure?

Dear All,

I am using BEx Query Designer. I made three date variables on 0netduedate(net due date) , 0pstng_date(posting date) and 0clear_date(clearing date). By inputting the netduedate at selection screen , that same date should go to posting date and clearing date automatically. For that I know I have to user exits but unfortunately I am new to BW and CMOD and enhancements. So could anybody tell me the procedure how to use CMOD and build a relation between these 3 dates?How are these user exits connected to our infoobjects and their variables in query designer?

Regards,

Ratish