I've followed that guide before, after trying out a custom ROM and deciding to go back to stock. I had no problems; I thought it was pretty self explanatory in how to do it. What version of Moto G do you have? Can you get into fastboot still? Turn off phone, then holding vol up/down and the power button for several seconds. After a few seconds (maybe 10) let go and you should be in the bootloader. Then you can plug in the phone and your computer will recognize it in fastboot mode.

There are several tutorials out on the web about fastboot, and they most all will apply even if its not for the Moto G specifically. The guide has the commands to run. If you already have the boot loader unlocked (and based on your post stating you are stuck at that screen it appears you do) you don't have to use mfastboot (Motorola fastboot), you can just use fastboot.