Can I flash an RUU coming from Cyanogen on an HTC One M8? I have an HTC One M8 Dev edition running CM12. I also have TWRP and S-OFF with unlocked bootloader. I want to know if I can use the RUU for marshmallow for my phone and if there are any steps I have to do before running the utility
None... Just put your phone in fastboot mode and run the RUU. Although the Motorola flasher in me always does a factory reset first for safety.