The X-axis is by far the trickiest of the three. You won't be able to route the belt over the pulley by going in from the side of the x-carriage, so you'll need to go through the hole in the top. This works best if both ends are first curled before placing them through the top hole. Feed one curled belt end between the pulley and smooth idler, and instead of forcing the belt through, slowly move the x-carriage in the opposite direction to help reveal the end of the belt. Once revealed, grab the belt and pull it further through. Repeat this process with the other end of the belt, and center the belt such that there is overlap on each side of the X axis.