Need help flashing boot.img

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http://forum.xda-developers.com – I just got an HTC One X, and it's already been rooted. The recovery is TWRP and that's all I know. I'm trying to flash a ROM, but first I need to flash the boot.img, and that's the problem. I can't. There's an automated process, and I followed the steps, got into fastboot usb but the program just sits on waiting for device. Also, when I boot into fastboot it only shows fastboot in red, until I move down towards power off, then it changes into fastboot usb. So, I have no idea what's going on, or if I'm missing something since I wasn't the one who rooted it. Any help would be great, thanks an (HowTos)