How to restore the stock firmware to the Asus Zenfone 2?

Asus Zenfone 2 (Z00AD, ZE551ML) I have downloaded the stock firmware from here. But how to flash it? This guide tells me to put it on an external SD card and boot into "update mode" by pressing Power + Volume Down, but that combination doesn't do anything on my phone. It looks like I can use something called Asus Flashtool, but it appearantly involves using a different type of format for the firmware, which I will need to download from a third party. Asus has provided me with the firmware. How to flash it?