⬢Turn the frame on its "right" side, as shown in the picture. You need access to belt holder, idler and pulley.
Use the bottom opening in the extrusion and gently insert the cable from Y-motor.
DON'T TRIM THE BELT unless you are asked in the instructions!!!
⬢Find the belt for Y-axis (Y-GT2 650 mm) and insert one end in the Y-belt-holder. Ensure you are using the upper slot (closer to Y-carriage).
Make sure you won't accidentally use the belt for X-axis (X-GT2), which is longer.
⬢Press the belt all the way in, use Allen key.