Before getting started you need will need a USB Stick which can be fully erased.
Create a folder called Patchstick on your desktop, download atv-win from here, and place it into the folder.
Double click to open the atv-win-3.0.zip archive and drag the atv-win executable into the Patchstick folder.
Double click atv-win-3.0.exe to launch the application.
Select the newest firmware version from the Firmware Version dropdown and select SSH from the checkboxes below. Click the Create Patchstick button to continue.
Atv-win will now ask you for a location to save a .img file. Navigate to the Patchstick folder on your desktop, input a filename of atv-patchstick.img and click the Save button.
Atv-win will download the latest AppleTV firmware build the .img file.
A popup will appear informing you that your patchstick img has been created successfully. Click the OK button.
When asked if you would like to run USB Image Tool now click the Yes button.
Select your USB stick from the list of devices on the left and then click the large Restore button.
Navigate to the Patchstick folder on your desktop and choose the atv-patchstick.img file we created earlier.
When asked to confirm the restore click the Yes button. Note: You will lose all previous information on the USB stick.
USB Image Tool will now create your patchstick using the img file we generated earlier.
Once the restore is complete eject the USB Stick and connect it to your AppleTV.
Hold down the Menu and - buttons on the AppleTV remote until it reboots.
Notice the Patchstick will now do its job and install SSH on the AppleTV.
When complete, remove the USB Stick from the AppleTV and unplug the device. Plug it back in to reboot.
You can now access your AppleTV via SSH to perform a host of functions and hacks. You can find many interesting tutorials for your AppleTV here. The default username and password for your AppleTV is now frontrow and frontrow.