It's a 'hair pin' clip. Once you have the bulb lined up in the hole, all you have to do is squeeze the clip, slide it in place and release it.
The hardest part is getting your dern hand in there. Whenever I replace the bulbs, I slide the tanks out of their holders to get them out of the way. I NEVER could replace bulbs with the tanks in place. I physically cannot do it. My hands are too big.
As said above, once you do it a couple of times it's easy. I can replace my headlight bulbs in 2-3 minutes now...
