Unlock/Root AT&T HTC M8

[sal2726]

I have no experience at this. My m8 is on 4.4.2. It will not fetch the OTA update, and the HTC RUU just sticks on sending. I would like to side load Marshmallow, but know nothing about the process. Is there a website with instructions for dummies? What software I need on my laptop, how to unlock bootloader, twrp, firmware? It's a secondary phone, if I ruin it, I'll live. I've read different articles, but I need a from scratch start to finish description. Any help appreciated.