Step by step pictures or video for those who likes visuals.
1. Launch Safari on your iPhone and go to iphonenodata.com
2. Tap the “Disable EDGE/3G” button
3. Tap “Install”
4. Tap “Install Now” to confirm
5. Enter in your passcode
6. Profile is installed and EDGE/3G data will be disabled!
To re-enable data and revert back settings to the way it was before, go to Settings – General – Profile