⬢Move the Y-carriage all the way from you.
⬢Using a finger on your left hand push the belt down. Some force should be needed for bending the belt, BUT don't try to overstretch the belt as you might damage the printer.
⬢Release the screw, parts will move apart, the overall tension will decrease.
⬢Use the technique described below to test if the belt is properly stretched.
⬢Use pliers to hold the Y-axis motor shaft.
⬢Move the Y-carriage with your hand towards the Y-axis motor. Don't use excessive force.
⬢If the belt is stretched properly, you should feel a resistance and the Y-carriage won't move at all. If the belt is too loose, it will deform (create a "wave") and jump over the teeth on the pulley.