If you are not familiar with the term Custom ROMs, let me clear things for you. Custom ROMs are modified versions of Android operating system based on the source code released by Google and/or device manufactures.
BEFORE YOU BEGIN
HOW TO FLASH CYANOGENMOD 13 FOR GOOGLE NEXUS 6
Download rom and copy the zip file to the root of your SDcard.
Power OFF your GOOGLE NEXUS 6 and wait for 10 seconds.
Reboot the device in recovery mode:
Press and holdVolume Down key and Power Key simultaneously. When the white bootloader appears, choose ‘Recovery’.
In TWRP recovery perform a full data wipe / factory reset.
Use TWRP to install the rom .zip file. Just tap Install, choose the .zip file, and the app will do the rest.
Reboot the smartphone when you’re finished.
NOTE:Flash GAPPS and WIFI Dialer if it comes with rom, in the same way as you flash custom rom.
You have successfully flashed your GOOGLE NEXUS 6 with custom rom.