This is How you install Paranoid Android on Nexus-6P.
Paranoid Android is the old king of custom Roms, it gave us many custom Roms for devices such as the Galaxy Note 2 and many more..
Read further for download links:
Download Paranoid Android
Download Gapps for Marshmallow(These are Google Apps for eg playstore,google play services etc, They do not come with the Rom)