[Nexus 6 Q&A, Help & Troubleshooting] CapitalParallax:
The above guide is pretty noob friendly for setting up adb and fastboot to your path. Since I recently learned this myself, I'll try to explain what you're doing. I will use [brackets] to show what you might type on your computer. You will not include those brackets when you do this.

For the sake of keeping things simple, make a new folder in your C: drive called "shamu".

You need to know some basic stuff with command prompt. For one, it doesn't really like spaces in file or folder names. Use an underscore for any spaces in any folders or files that you name. It is also case sensitive. So be aware of that. When you first open it, it is probably looking at a "documents and settings" or some similar folder. If you type [cd..] and press enter, it will go back one folder. You can probably do this two or three times and end up looking only at your C: drive. To navigate to your new shamu folder, type [cd shamu]. If you did this from <C:>, you should now be looking at <C:\shamu>. Think of [cd] as "change directory" to help you remember this.

Think of adb and fastboot as functions of the command prompt. It is how your PC talks to your device. When your device is in fastboot mode (the little dude, a bunch of little text, and a menu that makes use of the physical buttons), you tell command prompt to use fastboot to complete a task. When you type [fastboot devices] you are telling fastboot to list connected devices. This command is a good first step to verify PC and device are talking to each other. ADB is essentially the same thing, but you use the ADB function when your device is either in recovery mode, or booted up to Android. Whether you use adb or fastboot is dependent on the current state of your device.

When you set the path in the environment tables (as the above guide shows you), you are enabling command prompt to use fastboot or adb no matter what folder it is looking at. If you do not do this, you must navigate to (have command prompt look at) the folder that contains ADB and Fastboot in order to run it. Setting this up will make your life a million times easier.

Some adb and fastboot commands need multiple levels of instruction. If you are flashing an .img, it needs to know where it is flashing it to, and what it is flashing. You set up the PATH and made that "shamu" folder to make these steps easy and simple. Just as an example, when you get to flashing recovery, you will type [fastboot flash recovery recovery.img]. "fastboot" tells command prompt to use the fastboot function. "flash" tells fastboot to install something. "recovery" tells fastboot what part (partition) of the phone it is flashing to. Finally, "recovery.img" tells fastboot exactly what file it will flash to the partition you just specified.

The .img files you downloaded are all the necessary parts your phone needs to work. You must flash (install) them each. I am not sure if the order of operations matter, but I have been following the same order each time with success.

Do not try to move forward until you have the PATH thing working. Also, you should really have full battery before you start.

Copy all the image files to your new shamu folder. Boot your device into fastboot mode, connect it, and run [fastboot devices]. If it shows a serial number, that means you are good. Now, in command prompt, navigate to <C:\shamu> like I explained earlier. Because you set the PATH up, you can be in this (or any) folder and run fastboot commands. Since command prompt is looking at this folder, you will not need to specify where your files to be flashed are located.

First, flash the bootliader. In place of where I am going to type "bootloader.img", copy and paste the full file name. (In command prompt, CTRL+V does not work. You must right click and select " paste"). The file will contain the word "bootloader" but otherwise be obnoxiously long, like the radio file. [fastboot flash bootloader bootloader.img]. You just told fastboot that it is going to flash, on the bootloader partition, that particular file. Some stuff will pop up on command prompt, then it will say completed, and you can type again.

Now flash the radio. [fastboot flash radio radio.img]. Same thing, make sure you copy/paste the file name in place of "radio.img".

Now you're going to reboot your device directly to the bootloader again. [fastboot reboot bootloader].

Once back up (probably very quickly), you can now flash recovery. [fastboot flash recovery recovery.img].

Next up, boot.img. [fastboot flash boot boot.img].

Then, system. This one is going to take a while. Be patient. It doesn't look like it, but until you can type again, it IS working. [fastboot flash system system.img].

So far so good, right?! Now for the cache. [fastboot flash cache cache.img].

Almost there! This step will be slightly different. You're not going to flash the userdata.img. You're going to have the device create its own. This difference is especially important if you have a 64GB device. [fastboot format userdata].

Now, use the physical buttons to reboot your device. First boot will take a little bit, but you should be golden at this point. Good luck!