SQL & PL/SQL :: Making Audit Trail

I want to make Audit Trail on every DML operation users does on record. I have many more tables but I mention some example tables.

My Tables are

Invoice Table

Ref_Number char(12), -- Auto Generated
Invoice_No char(12),
Date Date,
Date_Created Date,
Created_By char(50),
Modify_By char(50),
Date_Modified date,
Amount number;

Receipt Header Table
Receipt_No Char (12),--Auto Generated
Date Date,
Date_Created Date,
Created_By char(50),
Modify_By char(50)
Date_Modified date;

Receipt Detail Table
Receipt_No Char (12),
Serial_No,
Doc_Number Char(12),
Date_Created Date,
Created_By char(50),
Modify_By char(50),
Date_Modified date;