You should have 1 long belt. Start by cutting it in half. It doesn't have to be exactly in half because you will have extra belt length when finished.

Feed the belts through the belt guides on the rear plate. When the rear plate is installed the belt teeth fill be towards the X 2020 extrusion.

If you don't want to use superglue move to the next step.

For added security I like to use a few drops of superglue(I use Loctite Gel). Fold the belt over so the teeth interlock. The belt only needs to overlap no more than about 1 inch. Using a pair of needle nose pliers to hold the belts for about a minute to give the glue time to set.