First, make sure you are in fastboot mode. The easy way would be connecting your phone to your PC via a USB cable with USB debugging enabled.
Then, simply open CMD from the directory where you installed ADB (or just open CMD if it's installed globally).
Then make sure ADB detected the phone via
adb devices. If your device shows up, just type
adb reboot bootloader and the phone will reboot to fastboot mode.
After that, open CMD and use
fastboot devices. If you get your device, you can proceed with unlock process, but if it doesn't return anything or sits on "waiting for device", then the first thing you should do is to try to plug USB into another USB port (for some reason fastboot is a massive pain in the ass when you try to use it on USB 3.0 port, so if you have USB 2.0 port try to plug USB into it, this is trouble only on Windows). If you don't have USB 2.0 port, then you should consider using Linux live USB to unlock the bootloader. However, if you plug the phone into USB 2.0 port and still is not detected, then you should try some other USB drivers, from the manufacturer's site or universal ADB drivers. Here is the link for various phones, so check it out.
Once, I managed to solve this by manually changing drivers (it's possible on Windows 7 and probably later, I can't tell the exact process since I don't have a PC currently). I chose something like "LG USB fastboot" because my phone is LG.