Try a Google search or other search tool? The first item that popped up for me was in
message #237 in the
Clara SD card image needed thread. You might also check
message #151 in the
Memory Upgrade Mod Guide (With Pictures) thread.
You will need to use a hex editor on the image file that you downloaded and enter your serial number as per the instructions given in those messages. I've attached an image of a Glo image file open in HxD (note much of the serial number in that image is randomized). Once you have entered the serial number and saved the image, you will need to write that image on a μSD card and install it in your Kobo.