I know it can be frustrated, but that's one of those things where you REALLY need fastboot setup and working before installing the Rom. If something weird happens where you're in a bootloop or something, you need access to working fastboot. If you can't get it working I'll help you get it figured out.
But tell us exactly what's not working and we can probably help.
One tip is if you're using Windows 7, open the command prompt by holding shift while right clicking in the blank white folder space where your fastboot.exe file is located on the computer.
Shift + Right Click will pop up a menu where you choose "Open Command Window Here"
That puts you right to the step of typing the command, because cd (change directory to path of fastboot file) isn't needed since you started in the correct folder.
When you run the command, like "fastboot devices" as a test, take a screenshot of the cmd window after you run it so we can see exactly what it's doing and advise from there.
If you're own Windows 8 you'll need to disable driver signing, which I find the steps for by googling "Windows 8 how to disable driver signing"
Also if there's USB 2.0 ports on your computer use them instead of 3.0. And the main USB port usually works best, such as on the back of a motherboard instead of the front case in a desktop tower.
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Read until your eyes bleed, and search until your fingernails fall off. ~ CajunFlavoredBob