How do I get my phone to boot into fastboot mode?

[LynxPavo]

Some preliminary information:
Samsung Galaxy S5 Verizon (SM-G900V)
Android Version 6.0.1

Basically, this all started out with me wanting to install lineage mod on my device. I learned that I had to unlock the bootloader. I found that I could do it with a simple command: fastboot oem unlock. So, I tried to get that to work. At first I couldn't even get adb work but after installing some drivers it worked. Then I tried to get fastboot to work but for the life of me I can't figure it out. I've tried Volume Up + Power, Volume Down + Power, going in to recovery mode, going in to bootloader mode. Nothing worked. Fastboot never detected the device when I ran fastboot devices. I even tried running adb reboot fastboot. No luck.

What am I doing wrong? While I'm here I might as well also ask if there is anything else I need to disable besides the reactivation lock in order to not brick my phone.

EDIT: Note, it's not just that the fastboot utility won't detect the device. I can't even get the device into fastboot mode.