Browner87 drilled a 12 millimeter hole into the ball, cut a ring around its middle and flattened its bottom. He then added an aluminum button to act as the Pokéball's clasp, sanded and painted everything before attaching it in his car. You can see a couple images of the in-progress and finished product above.
The ball apparently took about 30 minutes to make and another two weeks to paint. Check out the Imgur album for a look at more photos and his step-by-step process.