1. Download kali linux nethunter image file.
2. Now save the kali linux nethunter in root directory of nexus device.
3. Download TWRP manager application and hit the 'reboot option' tab and 'reboot into recovery mode'.OR alternatively power off your phone now.Press and hold Volume Down, Volume Up and Power, all at the same time to put into recovery mode.
4. Now start installing the kali linux nethunter via selecting the kali linux nethunter zip file.