Are you having trouble getting into Safeboot on the HTC One M8? Those guides that I used didn’t help me because the way they told me to use the button combinations didn’t work. The only button combination that worked for me was to turn the phone off and then hold volume down and power button at same time. Those other tutorials said to hold volume down first then hit power and go back to holding down volume down again, which never worked for me.
Getting into Safeboot mode is the first step in rooting and unlocking bootloader so that you can flash custom roms like I did. I just cannot stand using that BlinkFeed centered and bloat filled system that HTC put in this high quality smartphone.
There are plenty of guides online that you can follow to root and install custom roms.