Its pretty easy once get a blank USB stick and for your box if it is a cable set up you can use this image
Andy Mc OpenATV 6.0-beta ZgemmaH2 - Downloads - TechKings
all you do is drag the zgemma folder over onto your usb and insert it into the back usb port and power on your box the display will change from boot to flash and then once its done power down the box remove your usb and let it boot up and then you will have a working image all you will need to do is put your line on and nstall your harddrive