How flash ROM
First of all, you need to flash recovery for your mobile
1 Conect mobile with PC, open cmd.exe
2 Type following for fastboot
adb reboot bootloader
3 Type for unlock bootloader
fastboot oem unlock
4 Type for recovery twrp
fastboot flash recovery recovery_twrp.img
Turn off your mobile. Press 2 buttons together, power+vol in 10 seconds, mobile go into recovery.
If you want to root it, flash supersu.zip 2.71 over recovery!