Here's the trick.
Reboot to fastboot by pressing POWER and VOLUME DOWN together until you get to the first boot screen. Select "Recovery Mode" by using the volume buttons to scroll and confirming with Power. It will reboot into recovery (the smaller Android with the open chest and Red "!") Now - here's the tricky bit - you have to press POWER and UP together ONCE to enable the recovery menu.
Then, you can select Factory Reset and you're good to go.
You don't need to be plugged in to a computer.