Place in Internal Storage and flash in recovery.
MUST BE ON STOCK RECOVERY TO FLASH IT. If you flash it in TWRP and boot loop, without making a backup beforehand, you're on your own. You should've made a backup AS ALWAYS.
February 2016 Security Update OTA
Google Drive: blur_version.24.11.18.clark_retus.retus.en.us (flash in stock recovery ONLY)
Stock odexed ROM with the February 2016 Security Update applied. No root. Flash in TWRP.
Google Drive: stock_marshmallow_for_moto_x_pure_edition-mphs24.49-18-3 (flash in TWRP)
Modified boot.img post for use with traditional SuperSU root by DaMadOne here:
Stock 6.0 Marshmallow Recovery:
Google Drive: stock-6.0-mph24.49-18-recovery.img (flash in fastboot or TWRP)
Google Drive: stock-6.0-mph24.49-18-recovery.zip (flash in TWRP)
NOTE: If you get "No command" while booting into stock recovery, hold the power button and press up on the volume key. Options should now be available. I'm not sure why it does that but it used to do it on Nexus devices (and probably still does.)