Refer to the belt routing PDF in the introduction. Make sure the belt doesn't twist. There's one spot each belt is meant to be flipped otherwise it should be straight.

Looking at the back plate(standing behind the printer) take the "lower" belt and feed it to the left

With the smooth side of the belt touching the bearing, go around the bearing towards the rear corner.

Loop the belt around the lower bearing. The smooth side of the belt will touch the bearing. Pull the belt slack through and towards the other rear corner.