1. Place the zip on your external SD card (recommended) or internal memory
2. Boot into recovery using NDR Utils or by pressing the volume down button on boot.
3. Flash the zip using TWRP, other recoveries might work but I have not tested it yet.
4. Clear dalvik cache and other cache.
Download link for D6503
Download link for D6502
Info about the zip
Operator: Customised SG
Get your own FTF file using XperiFirm in Flashtool.
Create your own pre-rooted firmware with your own region with PRF Creator.
Do feel free to post any problem you might have.