1. Turn Off Your Phone Completely [ Skip This Step If Your Phone Already Off ]
2. Now Press and Hold Volume Down + Home Button and Power Button Togather.
3. In 2-3 Seconds U Will See Green / Blue Warning Screen With 2 Options ( Volume up to continue and Volume Down To Restart ).
4. Now Press Volume Up Button Once To Get To Download Mode Screen.
5. Done Now Your Phone In Odin Mode and U Can Now Write Firmware With Odin Tool.
Related Stuff :-