To flash MegaTron you need to be rooted.
You will need this ---->
Odin
and this ----->
Transform.ops
and this ----->
build7
Download those first. Then:
(thanks to mavrikmeercat as I forgot this simple step)
You need to enable usb debugging:
Go to settings, applications, development, and tap on usb debugging. Usb debugging should have a check mark in the box now.
It is a good idea while your there to tap on "stay awake" too. This will enable your device to "stay awake" while charging or connected via usb to your computer. Some programs require the device to "stay awake" while programming, so it's just a good idea to go ahead and enable.
To get into recovery:
1: Phone off
2: Press and hold the Volume down button
3: Press and hold the Power button
4: When Samsung logo appears let go of the Power Button
Take cover off back of phone. Connect phone to computer via USB. Once connected pull battery.
Once in recovery:
1. Run the ODIN program you just downloaded.
2. Put your phone into download mode by pressing and holding the camera + volume down + power button at the same time.
You should see a screen pop up that states you are in download mode. Press the volume up button to continue.
3. Hook phone up to computer via usb. On the ODIN screen you should see a yellow box appear in the COM Port Mapping (about 1/4 of the way down on the left) that shows you are connected on COM6 or something similar.
4. Below COM Port Mapping under Option you will see 4 options:
One Package
Auto Reboot
Protect OPS
Reset Time (Sec)
Make sure the first (3) are checked and Reset Time (sec) is unchecked.
5. Below Option under the Debug Option you will see 1 option:
Debug ONLY
Make sure and have that unchecked.
6. 1/2 down on the right you will see "Select OPS".
Click the "OPS" button and find the file you just downloaded: TransformUltra.ops
7. On the bottom right you will see "Selet Integrate Package - Check One Package Option". Click "One Package" and find the file you just downloaded tu_system_boost.
8. Press start.
Let it finish.
9. Follow steps 1-7 again.
10. In "One Package", find the file you just downloaded "tu_stock_recovery" and hit start.
Let it finish.
11. Unplug phone and boot up.
You should be rooted and have CWM installed in order to flash Megatron.