Its a little involved John, as this is essentially flashing a ROM onto a Android device. It is a little on the techie side of things.
I had a set up from a previous android box which included a USB hub and wireless mouse. Keyboard is not required, so if you have those handy it makes life easier and less expensive.
Once you install this ROM you interface via a mouse (the Nexus Player remote will have limited functionality, essentially up down left right and enter for apps supporting it). I use a standard wireless mouse with the dongle plugged into the USB hub.
If you are okay with that, then you can continue. I used my laptop to install the apps below and then you connect the Nexus Player to the TV via HDMI and your laptop via USB (your Android phone usb cable will work if you have one, otherwise you need a micro USB to USB cable). When you run the first command adb devices your Nexus Player will prompt you to confirm authrozation (you'll need the Nexus remote to accept).
Here is a guide
Flashing Guide - Android - XDA-Developers
The first thing is to get the Nexus Player ready, you need to enable developer options on the Nexus Player, by clicking the build number 7 times (with plaer remote) or so til it says its in developer (under settings about Build Number) mode. Once you have done this you need to enable USB debugging, this allows you to use the USB port of the Nexus Player. You can find this in the developers option under settings of the Nexus Player.
Now you have to download the Android SDK in order to get the ADB and Fastboot programs, this is what is used to flash ROMs onto android devices.
Download Android Studio and SDK Tools | Android Developers
Once you have the SDK installed (make sure you know the path of where it is installed, I installed to desktop, as you will need to know it to run ADB and Fastboot from CMD ) Below is a guide you can start at #1. Unlocking Bootloader
[GUIDE] Nexus Player - How to Unlock Bootloa… | Nexus Player | XDA Forums
These are the commands I used and sequence.
1. ADB devices (your Nexus Player will prompt you to authorize the access, use the remote to accept. You're ready to flash)
2. ADB reboot bootloader
3. fastboot oem unlock
4. fastboot oem unlock (second time)
5. fastboot flash boot boot.img
6. fastboot flash system system.img
6. fastboot -w
You will need your mouse plugged into a USB hub that is connected to the Nexus Player in order to operate.
It took me several attempts to get this right, which I have explained here ( had to go get the answers through forums, etc) so it should be easier for you.