I'm following the instructions at http://htc-one.wonderhowto.com/how-to/unlock-bootloader-root-your-htc-one-m8-0154444/ and have found a stage where I don't have enough information, which is at Step 9 in the instructions, which says "Start by grabbing SuperSU from Chainfire directly on your phone, or download and place the file in your device's "Download" directory."
I tried doing it directly on the phone, but that didn't install it properly --- it seemed to want the phone already rooted, but I'm doing this as a stage of rooting the phone. When I then try running supersu, it complains it can't find the su program.
So now I'd like to "place the file in the Download directory", which presumably means using "adb push", but I can't find where I should be putting it, i.e. any directory called "Downloads" or anything like that. I've looked around, using "ls" in the adb shell, and I suspect it might be in /data, but I can't look in there (perhaps because the device isn't rooted yet?) The instructions expect the supersu zip file to be unpacked by the TWRP recovery, which I have installed.
http://androidadvices.com/root-install-custom-recovery-twrp-image-htc-m8-android-phone-guide has alternative instructions, which just say to put it in "a suitable location"; could I, for example, just dump it in "/" ?