Where to put supersu zip file while rooting HTC One M8
Download the zip to your computer, boot up the phone in normal mode (just press the Power on button when turned off - no bootloader, fastboot or anything like that) and connect it to the computer via USB. The phone memory that opens up on your computer is where you should put it. It really doesn't matter the exact location, as long you can access it later in the TWRP recovery (the Download directory mentioned in the guide is also there). Then you can proceed with Step 10 (or Step 15 if you have already installed the TWRP recovery).