Step 1 Prepare for Installation
Reset the AppleTV's HDMI Connection
Before you install OpenELEC it is good practice to reset the AppleTV's HDMI connection to "RGB High" or "RGB Low" as the AppleTV video hardware sometimes forgets the HDMI output setting (even with the Apple OS, and sometimes even if you did this step) resulting in flashes of neon pink during the initial stages of OpenELEC booting. This option can be found on the "Audio & Video" settings sub-menu as shown below on v2.1 AppleTV software (v3 is laid out differently but the menus have the same options):
If the flashes of pink bother you, the solution is to perform a factoryrestore boot and reset the HDMI connection, then reinstall OpenELEC. If you made a backup of your configuration first (assuming that you do not store media on the internal drive) the backup and subsequent restore can be done from the OpenELEC settings app.
Connect the AppleTV to the Network
To keep the installer small (about 12MB) it does not include the OS build files in the installer image, it downloads them from the internet as part of the installation. This also ensures you always get the latest build. Before you continue, please connect the AppleTV to your router with an Ethernet cable. When the installer boots it will lease an address via DHCP before downloading the image.
Find a USB key
You will need a USB key 512MB or larger to make the installer USB. It does not matter whether it is USB2 or USB3 or how it is currently formatted.
Make a Cup of Tea or Coffee
Things often go easier with a nice cup of tea or coffee. Go make one then continue to Step 2 using the links below!