The black timing belt goes through the flat disc with the rectangular hole, then is pressed into the bolt clip.

Notice how the belt if fitted to the last notch of the bolt holder. This is important to ensure accuracy. It is OK to press the bolt holder against a hard surface to get the belt fully seated in the gap.

Click the bolt into the bolt holder.

Note how the hole in the switch disc is rectangular. Press the switch disc onto the bolt holder with the long side touching the belt. It should be a snug fit.