1. Don't know why you think that. Just look in Original Development: http://forum.xda-developers.com/att-...ig-development
To flash any custom ROM, the basic sequence will be:
- Unlock bootloader via HTCDev.com
- Install custom recovery
- Transfer desired ROM to device
- Boot into recovery and flash the ROM
There is an excellent step-by-step guide here:
But since your goal is to install CM (instead of just root the stock ROM as the guide intends) you can stop at Step 8 of "Rooting Your Device", and instead transfer the CM ROM to your device and flash from recovery (similar to how the guide instructs to flash SuperSU..zip).
2. All the custom ROMs for this device are pre-rooted.
3. Odin is a root tool for Samsung. It does't apply or work in any way on HTC, so you can completely disregard in terms of this device.
Also, all questions should be posted in the Q&A forum section. I've reported to the mods to move it.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)