Make a backup!
If you were on CM 10.1 or a stock ROM prior to 4.2, upgrade to stock 4.2 first.
Install ClockworkMod recovery with SELinux support (.tar) (.zip). Older CWM will result in a "status 7" error.
Reboot into recovery
If you are using a recovery which mounts /system on boot, unmount it before installing
Wipe data if coming from stock or other ROMs (You may get a "status 7" error if you fail to wipe.)
Install the ROM
Optionally install the Google Apps Addon (In Marshmallow, you MUST flash gapps before booting into the system for the first time or else the proper permissions will not be granted resulting in Setup Wizard crash. If you experience the crash, you will need to do a factory reset.)