I’ll post some images where you can see how the belts are attached to the printed parts and how you have to place them around the pulleys. Click on an image to see the larger original file.
Place the belt inside the belt holding pockets, the belt teeth in contact with the serrated part of the printed part.
Belt around the GT2 pulley, with the belt teeth in contact with the pulley’s teeth. The belt is hold in place by the two pair of bearings. The correct position of the belt is where it passes above the endstop between the two bolts.
The Y belt is attached to the printer head part by introducing the belt in the belt holding holes and securing it with a zip tie.
Just like the X axis, the belt teeth should be in contact with the pulley and it should pass above the endstop.
Place the belt inside the belt holding pocket with the teeth secured by the serrated surfaces of the printed part.