I removed the weather shield/shade from the lamp and was left with a fitting that resembled a standard room light. I then unscrewed the light fitting from the cable to measure a hole to insert the light fitting through.
I made a small hole, and then gradually enlarged the hole until the fitting could just about fit through with a push. The last thing you want to do is create a hole that is too big, so cut-and-check until you get the right size.
The lamp was then fitted and the the cable end was screwed back on to hold it in place. For a safety measure, I also put the bulb cage back on that came with the lamp – hopefully this will stop the bulb from touching the cardboard and possibly causing a fire.